Avocets, terns and gulls swoop down onto Wallasea Island on England’s eastern coast, searching for food between blades of grass ruffled by the summer sea breeze. Aside from the wind, and the odd chirp or squawk, it’s quiet—the kind of peaceful scene that seems like it’s been going on for centuries. Yet five years ago, these wetlands didn’t exist.

The mud the birds have landed on once lay under the streets of central London. In 2015, as part of a railway project, a construction crew scooped more than 3 million metric tons of dirt out from beneath the capital and piled it onto farmland on the coastline of the county of Essex, 50 miles east. In summer 2019, a crane hoisted old heavy machinery out of the water, removing the last vestiges of human interfer…