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LONDON — Until two years ago, West London's Greenford Tube station used to flood whenever it rained heavily. The train tracks are aboveground, but the ticket office would often get inundated. Sandbags still line the corridor.

But in October 2023, a new family moved in nearby, determined to halt the water. The family members built their house from scratch with local wood and kept odd hours, sleeping all day and working only at dawn and dusk. They even put their young children to work.

The new neighbors were beavers.

The beavers are part of an unlikely effort to bring back a vanished species and help Britain adapt to a very modern problem: climate change.

Britain is famous for drizzle, but climate change is making rainfall heavier and more erratic. Places that didn't used to flood are now waterlogged. So scientists have enlisted some of the animal kingdom's best flood engineers — beavers — to help.

In West London, conservationists got a government license to resettle a family of five beavers in a 20-acre urban park near the Greenford Tube station. It used to be a golf course, with a creek running through it. Within weeks, the beavers dammed up the creek, creating a pond that holds water and stops it from spilling into the city. They also diverted the creek's flow into smaller tributaries, creating a wetland that better absorbs heavy rainfall — mitigating the risk of flooding downstream.

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"They effectively turned this site into a giant sponge that can take heavy rainfall and slowly release water back into the landscape, creating a lot more resilience for flooding," explains Sean McCormack, a local veterinarian who started the Ealing Beaver Project, named for the London borough of Ealing, where it's located.

Not only has the local Tube station stopped flooding, but the beavers have also coaxed back other species.

"By felling trees, they've also opened up the canopy, and we've seen an abundance of biodiversity," McCormack says.

Freshwater shrimp have appeared in the creek, he says, plus eight new species of birds, two types of bats and rare brown hairstreak butterflies, which lay their eggs on blackthorn branches nibbled by beavers.

The beavers have also allowed the city to scrap expensive plans to dig a reservoir and levee.

"We said the beavers can do it for a fraction of the cost, certainly more sustainably," McCormack says.

Now, joggers and teenagers stop to gawk at the beavers in action. There are guided walks and beaver safaris.

On a recent spring evening, a reddish-brown adult scampered in and out of the water, chomping on a felled willow tree. Eurasian beavers can weigh up to 65 pounds; this one was the size of a fat golden retriever.

The Ealing Beaver Project is one of dozens of sites across Britain where land managers are using beavers to restore wetlands and tame flooding.

But first, they had to bring them back from extinction.

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Reintroducing beavers to Britain for the first time in centuries

In Britain, humans hunted beavers to extinction more than 400 years ago. By the early 20th century, only about 1,200 native beavers were left in Europe and northern Asia, surviving in parts of Norway, France, Germany, Belarus, Ukraine, Russia, Mongolia and China. Sweden reintroduced them in the 1920s, and other countries followed — part of a broader effort to restore native species.

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By studying fossils, scientists determined that today's Norwegian beavers are genetically most similar to the beavers that lived in Britain centuries ago. So in 2009, wildlife officials relocated two Norwegian beavers to Knapdale Forest, a temperate rainforest in western Scotland. That pair, named Millie and Bjornar, became the Adam and Eve of the modern-day British beaver population. The Scottish forestry department calls them the "original beaver power couple."

"We became kind of attached to Millie and Bjornar," says Pete Creech, a forest ranger who remembers when they arrived, scrambling out of crates and splashing into a loch. He recalls their enthusiasm: "Lots of squeaking!"

Creech set up hidden cameras to capture their crepuscular (dawn and dusk) movements. Within weeks, the beavers dammed up a tiny river, creating an enormous lagoon where swans now nest.

While the United Kingdom overall is getting wetter, some areas — including parts of Scotland — are getting drier, even seeing a growing threat of wildfires. Beavers ensure this rainforest stays wet and, thus, abundant. That's especially important at a time when wetlands are disappearing, with many drained for development.

"Wetlands are one of the most biodiverse habitats in the world," Creech notes. "The U.K. has lost over 95% of its wetlands, and now we're frantically trying to put them back."

Not everyone thinks rodents are the best way to do that, though.