It’s not the most efficient way to cook. But we went to Vashon Island just west of Seattle to understand how cook stoves are actually getting better—and improving a whole host of other issues as they do. “Cook stoves are really a nexus for every environmental problem you can imagine,” Peter Scott told us. A more efficient stove means that young girls don’t have to go collect wood or coal as often, leaving them more time to get an education and advocate for change in their communities. A more efficient stove also alleviates poor indoor air quality, a problem that kills about two million people a year, according to the National Institutes of Health. Finding a way to make coal burn cleaner not only limits deforestation, it also helps cut down on greenhouse gases that are warming the planet.

Burn Labs founder Peter Scott wants to create the world’s most efficient cook stove. Photo by Spencer Millsap / NGM Staff

A few years ago, Scott started a company called the Burn Design Lab. He had spent some time living in several parts of Africa and wanted to create the best cook stove possible.That meant making the heat last a long time with minimal fuel. It’s a constant process of innovation. One of Burn’s stoves, known as the Jiko Poa, is about 45 percent more efficient than cooking over an open fire.

What makes a good cook stove? The design of the stove has an impact on how much heat is retained. The hotter it stays, the less fuel needed at the onset. Natural gas may be an abundant and efficient way to keep a fire going, but many countries in the developing world don’t have adequate fuel distribution lines. While that problem gets worked out, people still need to cook every day.

To meet the needs of everyone who needs a better stove, there is still a way to go. For the roughly 2.5 billion people around the world who still cook over open fires, Burn Labs is aiming to distribute 3.5 million stoves. The stoves are designed to be made for $15 and sold for $40.

The day we visited Burn, one of its top employees was getting ready to board a plane to Kenya. One of the newest prototypes had an eager audience waiting near Nairobi. The power of the stoves isn’t in the stoves themselves, Scott told us. Their real potential is what else people can accomplish when they don’t need to worry about stoves at all.

Part of the answer may be under the streets, down in caverns more than 200 years old that were excavated for coal. As coal mining became too expensive in Glasgow in the 20th century, the mines were abandoned. The pumps that kept water from trickling in were shut off, leaving the tunnels to flood.

Now, below the feet of Glaswegians, there’s are more than a million gallons of water sitting in the caverns, heated by the Earth. No matter how cold it gets in chilly Glasgow during the winter, the water still stays fairly warm by comparison, about 51 degrees F (11 degrees C) near the surface. The deeper you go, the warmer it gets.

To understand why that matters, we talked with Diarmad Campbell, an energetic Scot with a fantastic title. He’s the chief geologist of Scotland, working for the British Geological Survey.

The water is dirty, but the heat it holds is valuable. Geothermal energy is often discounted (or, at the very least, not considered part of the renewable energy equation) because it’s simply not visible the way you’d find solar panels or wind turbines. But the heat in the water can be potent, and Campbell’s team estimates that the energy in the water could meet the heating needs of more than 40 percent of Glasgow’s homes and businesses.

To get there, the first step is to map the caverns to find out just where the water is. Then the city would build pumps at four different locations to extract the water out. At the surface, it would be run through a heat transfer device that would remove the heat. Then the cool water would be put back in the ground, where it would heat up again.

Other countries already do this. Germany and Sweden have invested in this technology. According to one estimate, there are 10,000 heat pumps installed in the UK, and many more around the world.

But Glasgow’s seems a bit more enchanting because of the haphazard way the idea came together. “It’s by accident that we’ve created this opportunity,” Campbell told me. No one could have known 100 years ago that coal mining would help power the city centuries into the future. Let alone on renewable, clean energy.

There may be an even more profound way for the oceans to forgive our mistakes: by providing us lots of food. That’s the premise of a book that came across my desk, The Perfect Protein, by Andy Sharpless. He heads the ocean conservation nonprofit Oceana. There’s a lot of untapped potential swimming all over the planet. If only we managed the way fish are caught, and which fish we eat, there’d be plenty of food for the growing number of mouths on the planet.

If that sounds fanciful, it did to me too. Fish aren’t thriving; many of them—species like Atlantic cod and bluefin tuna—have been decimated by overzealous harvest. Countries with lax regulations on catch quotas effectively pillage the seas. Nearly every week I hear about another international conference where some countries are pushing others to stop abusing the ocean to let it recover.

Author Sharpless tries to get above the country-to-country bickering to make a simple point: Countries don’t necessarily need to agree. Most of the world’s edible fish aren’t in the high seas. They’re within 200 miles off coastlines, areas that are generally controlled by only one government. If about ten large countries (the U.S. would need to be a big player) set some basic limits, there would be plenty of fish to go around. And there’s good reason to believe that as America goes, other countries such as Chile, Brazil, and even fish-loving Japan may follow suit.

Still, the bigger change might need to come from addressing a sizable inefficiency in aquaculture. Fish farmers take highly nutritious, fast-growing species like anchovies, mackerel, and sardines and feed them to larger, less efficient fish like bass and salmon. Getting people to eat those small fish, which are packed with healthy Omega-3 acids, is the solution. And to help start the transition in a big way, Sharpless includes a few recipes from celebrity chefs like Mario Batali and Jose Andres to spark readers’ culinary creativity.

Whether you can get excited about sardine tartar or sautéed mackerel is a matter of taste, of course. But if food volume is the issue, and feeding more people around the world is the goal—and how could it not be?—Sharpless’s sound argument is that changing our tastes to eat more of those small fish (and less of the big ones) could lead to much more protein to go around.

Thanks to the Clean Water Act, oysters can now populate the river and start slurping dirt once again. The only problem is how to get them back there. This week Change Reaction visited the New York Harbor School, a small public high school on Governor’s Island, just south of Manhattan. The school’s goal is to return 1 billion oysters into the river over the next twenty years. It’s a small fraction of the number there once was, but it’s a start. And the school is using the project as a teachable moment, instructing students how to nurture the oyster larvae, build artificial reefs for the oysters to live on, and how to dive in order to put them at the bottom of the river.

Can it be done? Since 2011, the school has introduced about 2.5 million oysters. That number is expected to rise dramatically as students get faster at the process, says Murray Fisher, the founder of the Harbor School and the brains behind the Billion Oyster Project. They also hope outside funding from the city may allow them to build new equipment. “What I’m most excited about is having this type of environmental restoration become a good model for engaging students,” Fisher said, while volunteers counted oyster larvae nearby. The only thing definitely out of the question: At the end of their lives, the oysters are filled with so much bacteria and pathogens from a lifetime of filtering, they’re unsafe to eat.

Scroll down for photos.

Oyster larvae—known as spat—are placed shells and then put into the river to grow and mature. Photo by Spencer Millsap / NGM Staff

The process involves a lot of shuttling, taking boxes in and out of the river to monitor oyster growth. Photo by Spencer Millsap / NGM Staff

One of the most tedious yet crucial steps is to count the number of oysters growing on shells. Putting a billion oysters into the harbor requires growing them first. Photo by Spencer Millsap / NGM Staff

Eriskay, where the ponies are believed to have originated hundreds of years ago, is a tiny island in the Outer Hebrides—a remote island chain we visited north of the Scottish mainland. Limited habitat and inbreeding had dangerously imperiled the Eriskay ponies, known for their fine gray coat and diminutive stature. In the 1970s, only 20 ponies remained.

That’s when a group of community members saw the urgency to save the rare horse from disappearing forever. Shetland ponies and other breeds were brought to Eriskay to breed with the remaining mares. Some original purebreds exist, but not many. A large portion of the offspring are mixed breed. It’s not ideal, but it’s a small price considering what was at stake. There are now 420 known ponies with Eriskay blood.

It’s still not enough to ensure the current population can sustain itself. They’re still considered dangerously at risk by the Rare Breeds Survival Trust, a UK nonprofit that keeps records on threatened species. The community remains on the case to ensure the islands’ future will includes Eriskays. An organization known as the Eriskay Pony Society has established a detailed studbook to track every individual pony. A key part of the group’s work is very focused breeding schedules to ensure the population of remaining Eriskays stays spry and diverse.

Boyle is a conservationist in the Scottish Highlands with the Royal Society for the Protection of Birds. He had taken us to see some wildlife and to talk about what makes the Uists, a small chain of Hebridean islands, so unique. By the time we set up our cameras and started talking about farming methods and bird-watching, Boyle was getting antsy—and so were we—glancing over at the incoming tide. We piled back in the car and sped across the sandy channel toward the mainland. “I’ve never gotten stuck here,” Boyle said, staring straight ahead while water sprayed up from our spinning wheels. “But I’ve been close many times.”

Photo by Daniel Stone / NGM Staff

The first thing that you notice when you fly to the Outer Hebrides is that the group of islands off the Scottish mainland feels a lot more remote than it actually is. Most islands are only an hour-long flight from Glasgow, the medium-size city adjacent to Edinburgh, Scotland’s capital. Landing on one of the islands in a small plane on a small runway next to a small, one-room airport, you get the feeling that you might as well be in the middle of a vast ocean.

Most people haven’t heard of the Hebrides (pronounced HEB-ruh-dees), let alone been to them. There aren’t many direct flights, and the islands don’t have the exotic palm-tree-dotted landscape that you’d find further south. It’s so rare for someone from England to visit the Hebrides that they use the islands in an expression to describe somewhere extremely remote. “Don’t be bad,” your mother might tell you, “or I’ll send you to the Outer Hebrides.”

So that’s where our ambition took us, somewhere remote and yet still accessible. Somewhere inhabited for thousands of years, yet still wild. And somewhere imperiled on the planet—and not just by the usual things like climate change and rising seas. There are 500 islands that make up the Hebrides. The several dozen that have people on them are bursting with life but at risk of losing both their culture and their unique ecology, both of which are intimately intertwined.

To understand why, it helps to know a bit about the Hebrides’ geology. Like many places in the Northern Hemisphere, the islands were sculpted by ice sheets that repeatedly scraped over them, most recently about 11,500 years ago. The combination of the sandy, peaty soils that cover the islands and the region’s frigid weather fed by the jet stream has created a rich and spongy terrain called the machair. The machair is the backbone of the Scottish Highlands. For centuries it fueled subsistence farming on the outer islands. It still exists because it hasn’t been abused: In the Scottish tradition of farming—which is known as “crofting”—land is left fallow between plantings to allow it to rejuvenate.

Photo by Spencer Millsap / NGM Staff

Letting land recover from agricultural use isn’t a realistic model for much of the world, where a growing population constantly demands more food. But the practice has had impressive effects on the Hebrides. Vibrant, organic crops attract invertebrates like worms, and those worms attract birds. The islands have become a haven for bird species like corncrakes and corn buntings, which are being driven out of mainland Europe. A lack of predatory mammals has allowed the birds to thrive. On a hike one day we came across a nest of wader eggs in the grass. Almost anywhere else on Earth with predators like foxes or badgers, such delicate eggs wouldn’t last a day.

“People think that when you have areas as nature reserve you just take over an area of land and let wildlife get on with it, but the machair is completely different,” Boyle, the bird conservationist, told us. He’s something of a rock star on the islands, constantly fielding questions from visiting bird-watchers. The wildlife—including Shetland ponies and a rare breed known as Eriskay ponies that roam the islands—attracts a lot of ecotourists from the mainland, especially in the summer when the weather is moderate (it’s never actually hot that far north) and daylight lasts until 11 p.m.

Yet many of the Hebrides have struggled to sustain themselves on occasional revenue from the tourists who show up. One reason might be that many of the islands simply don’t attract many people. The flight from mainland Scotland to most Hebrides airports isn’t far, but it can be expensive. Once you arrive, only a few houses and herds of sheep cover the landscape. On the Uists, a one-lane road is the main artery of transportation. It’s tough to be in a hurry when oncoming traffic requires pulling over every few minutes.

Young Europeans who go to work on the islands for a summer like the quiet. “It’s a great place to save money because there’s nowhere to spend it,” our 20-something hotel concierge told us one night while pouring beers. A BBC documentary narrated by Ewan McGregor that aired last month called Islands on the Edge may have been the best PR the islands have ever received—although some locals thought it focused too much on the Hebrides being at risk of dying, rather than as a worthwhile destination.

Photo by Daniel Stone / NGM Staff

But it’s hard to ignore the fact that many of the Hebrides, including the group of three islands we explored known as the Uists, are in a sort of slump. The main culprit isn’t climate change, but a constant exodus of young people who head toward the Scottish mainland for a faster pace. Over the past two decades, the Uist population of 6,000 fell by more than a third.

To stem the flow, last year the Scottish government launched a $1.6 million program to identify the teenagers “most likely to leave the islands” and teach them vocational skills to encourage them to start businesses there instead of packing. “We hope to see more young people making a positive choice when they leave school to enter the world of work and business here in the Outer Hebrides,” the program’s organizer, Angus Campbell, told the local Hebrides News.

Failing to keep the islands populated would effectively mean giving up a piece of their ecological richness. No one around to cultivate the land would mean no more food for those earthworms that feed the birds—birds that are running out of places to go.

The day before we left the islands we met up with Johanne Ferguson, a biologist who works for Scottish Natural Heritage, the historical preservation program of the Scottish government. Her parents were born on the Outer Hebrides, and so were her grandparents. She took us up to a hillside that overlooked the stunning wild lands of the Uists. Then we walked down a small road on the way back to the car. “What makes these islands so special,” she said, “is that development and conservation efforts can work together.” In other words, the place wouldn’t be a good model for the rest of the world if it disappeared.

National Geographic videographer Spencer Millsap films a close-up of a Shetland pony on South Uist in the Outer Hebrides islands. Photo by Daniel Stone / NGM Staff

There are a lot of differences in how individual countries do things, of course. The metric system might be the biggest—the United States is largely on its own in measuring with feet and inches. Currencies and exchange rates are certainly country-specific as well, not to mention languages and customs. But all of those things go back hundreds of years. Automobiles arrived in the 20th century. Why couldn’t the world standardize one system for driving?

It’s harder than you might think. Despite the modern invention of the car, the side of the road on which we drive has a centuries-old history. The fact that most people are right-handed is the biggest factor contributing to which side of the road people initially chose. Ancient Romans drove chariots with the reins in their dominant right hands to allow them to whip a horse with their left. That way there was little risk of accidentally whipping a passing chariot. But if a warrior needed to do battle from a horse, he could attack a passing opponent on the right with his stronger hand.

For centuries, driving on a certain side of the road was mostly just a custom. There weren’t that many travelers and roads weren’t paved or marked to direct traffic, so it didn’t matter too much. But as more people started driving, some uniformity was needed. One of the biggest influencers of driving direction was Henry Ford, who designed his Model T with the driver on the left. That decision meant cars would have to drive on the road’s right, so that passengers in both the front and back seat could exit the car onto the curb.

Many countries eventually followed. Canada, Italy, and Spain changed to right-side driving in the 1920s. Most of Eastern Europe changed in the ’30s. Scandinavia waited until the 1960s, but its countries eventually changed to the right, too. Things got interesting in colonial countries, especially in Africa. France had long been a right-side country and Britain a left-side country, so their colonies usually followed suit. But when they became independent, many sought to normalize with their neighbors to make things easier. Today, most African countries drive on the right.

Driving on the left means sitting on the right. Photo by Spencer Millsap / NGM Staff

So why do close to 50 countries still drive on the left? The short answer might be stubbornness, which—we should be fair here—is part of the same reason the U.S. still sticks to measuring in inches and feet. But the more nuanced reason is momentum. Cities like London were designed to accommodate left handed driving, so switching would be no simple tweak. Changing the rules of the road is a very complex and expensive thing to do. And the more time that goes by, more cars on the road makes it even harder.

It’s certainly not a debilitating difference to foreign drivers. After a few minutes, your mind tends to adapt. But the most fascinating places to see the confusion might be at border crossings, where drivers are required to immediately change sides. British drivers who take their cars under the English Channel need to swap when they arrive in France. The same is true when crossing borders between China and Pakistan, as well as China and Hong Kong. Where possible, that seems like  a good enough reason to cross a border on foot.

The reason why is London’s curious urban design, a squirrely mix of streets that were designed over centuries rather than by a one-time urban design grid that you might find in New York or Washington DC. There’s no pattern to learn in London, or a system of mnemonics to remember the order of roads. You simply have to learn every street in the city. And before you can legally drive a taxi, you have to prove to a group of city officials that you can, without fail, navigate between any two points. During the tests, aspiring drivers have to dictate the most efficient route and recall landmarks they’ll pass on the way. The people who are very good at it—and let’s be honest, more than 90 percent are men—can master the system in two years. Most people take four or longer.

Photo by Spencer Millsap / NGM Staff

It’s a fun tourist novelty to know that the person driving you has a very detailed spatial map of the city in his head. But for about a decade, a group of researchers at the University College of London have looked into the effect that memorizing such a disorganized system has on your brain. The part of the brain that navigates spatial intelligence is called the hippocampus, a pair of two chestnut sized masses toward the back of your head. The researchers found that London cab drivers have uniquely bigger hippocampi than almost anyone else.

We asked a few London cabbies about this in hopes they could help us understood how their brains worked.

“Oh yeah mate, it’s called the hippocampus,” one cabbie named Simon told us. “Most people don’t use it because of the simplicity of navigating most other places and because of maps and GPS. But with London there’s really no other way.”

What’s it like to map something very complex in your brain, we asked?

“Well, right when the person asks where to go, it’s like an explosion in your brain. You see it instantly.”

An explosion in the brain is a pretty vivid image to understand just how someone’s mind works. Yet it rings true. Each time we got into a cab and stated an obscure street name or small neighborhood, the driver didn’t even respond. He just started driving, seeming to know immediately which streets to take, and what the most direct route would be.

The downside to having a big hippocampus is that when cabbies retire and stop using their spatial mapping so regularly, the hippocampus actually starts to shrink back to normal. It’s like a muscle that shrinks if you don’t use it. What’s more, memorizing such a detailed map of a sprawling city actually took up the place of other grey matter. Researchers found that cabbies were worse at remembering things based on visual information and had worse short term memories. There is, after all, only so much real estate in one’s head.

There are a few reasons to visit London’s sewers, or at the very least, think about them. The British capital has one of the oldest networks in the world—1,200 miles of underground tunnels built in the 1860s by a British engineer named Joseph Bazalgette. Through the industrial revolution and both world wars, those tunnels carried water and waste away from the constantly growing population of London. Now, after more than a century and a half of being taxed, the system needs some work. In the process, it’s also yielding a highly valuable and undeniably repulsive source of energy.

We were given a tour of one part of the sewer system by Thames Water, the public utility that manages all of London’s water infrastructure. We wanted to see how a sewer worked and to understand what was needed to retrofit the system underneath one of the world’s biggest cities. We were told to meet at a nondescript manhole cover in central London. And to bring plastic trash bags to cover our cameras. Mishaps, apparently, are quite common.

Despite what you’d think, it’s not only human waste that runs in the rivers of sewers. There’s lots of oil, especially after it rains and washes off the streets. There are leaves and dirt and garbage. We were also warned not to be surprised at the number of condoms that line the wall, stuck there by a fungus that thrives in the dark humidity.

What builds up the most is fat—cooking oil and grease that people wrongly wash down the drains instead of throwing in the garbage. Over time, that grease, which can’t be dissolved by water, amasses into a large pile. It’d be hard to think of a more perfect name than what sewer managers call them: fatbergs. A small mountain of built up fat smells about the way you’d imagine. When I reached out and grabbed some to feel its texture, several sewer workers looked on and simply shook their heads at my foolishness. Once you break the outer seal of a fatberg, I was later told, the stench multiplies.

Because of what they’re made of, fatbergs can actually be burned for energy. They’re highly caloric, which makes them able to be burned very efficiently for a long time. City engineers ordered up a new power plant in East London to incinerate the bergs. If all goes as planned, they’ll produce 130 gigawatts of power each year, enough to power about 40,000 homes.

The other reason we wanted to see London’s underground rivers was because of the way it’s changing for the future. Most of London’s focus and money at the moment has gone toward expanding the sewer system to accommodate a heavier flow of water, especially when it rains and dirty storm water overflows the system. With nowhere to put it, the polluted water is dumped into the River Thames.

London’s answer is a new bit of ambitious infrastructure called the Tideway Tunnel. It will funnel the overflow sewage to a treatment plant. “This tunnel might be the last change we’ll ever have to make to the sewer system, perhaps ever,” Rob Smith, technical coordinator for Thames Water told us. The utility’s engineers have said that when the project is done in 2016, it’ll be built to last for 100 years.

To the people who work in London’s sewers, the network of pipes can be a metaphor. The growing rivers of waste literally illustrate London’s growing consumption. The fatbergs symbolize a city’s clogging arteries. The expansion of the system signifies a city on the move, planning for a cleaner and more efficient future. Thinking about a sewer’s higher meaning, we learned, is a way to pass the time underground. But the smell is something you never get used to.

There’s a lot that’s old about London, but there is even more that’s new—ways we found that the city and its eclectic population are driven to make things cleaner, more efficient and with an eye toward the future.

Click through the photos above for a peek inside our reporting. And stay tuned to Change Reaction for more stories, photos and videos from all over the UK this month and next.