Two weeks ago, we spent a few days in Berlin and London. We met entrepreneurs pursuing the question of farmland, and whether there’s enough. On a planet with exploding demand for food, increasing population, and finite space, we wondered how the world would continue to feed itself.

In both cities the entrepreneurs—not farmers—that we met had some funky ideas. One was to farm in dark tunnels originally designed during World War Two. The other was a contraption you might one day find on your roof that uses an aquaponics system to yield fresh tomatoes, peppers, and live fish.

By no means are these the only new ideas in urban agriculture. Farmers in Brooklyn and Hong Kong are testing new projects. Detroit is