Light, seeds, soil, and water. The recipe for growing food out of the ground is among the planet’s simplest equations. Tweak the variables—less light, different dirt—and you can grow anything from feathery lettuce that’s lighter than air to a beefy jackfruit, the world’s densest fruit, which can weigh more than a hundred pounds.

So it’s strange that in the middle of the day on a recent Thursday we’re walking through a dark tunnel on the way to a farm. The facility is a hundred feet below the streets of London, 179 steps down, in an old air-raid shelter built in 1944 for protection from German bombs. During the blitz more than 6,000 people huddled shoulder to shoulder in this small tunnel.