Parcs Holman speaks with a rambling conviction about the mechanics and advantages of aquaponics. "It's totally the future," he says. "You're growing hundreds of pounds of food a year for a fraction of the water and power." Aquaponics, the growing of plants with roots submerged in water, is being touted by enthusiasts as the next frontier in urban agriculture.

The system depends on a network of tubes connecting a pond of fish with "rafts" that hold the plants. As the fish create waste, the water circulates to the rafts where tiny bacteria convert it into nutrient-rich plant feed. "It's just replicating nature," Holman notes. "Except nature has the sun, and we use a small amount of electricity."

Humans have been cultivating plants in water since ancient times. The Hanging Gardens of Babylon are said to be an early aquaponic system, and the Aztecs did it in the Valley of Mexico, where some communities are still growing food in such chinampas. Some south Asian countries also depend on aquaponic systems for rice cultivation.

What's different about this modern iteration is the scientific attention to detail. "There's a really delicate balance that a system like this requires. One tiny caterpillar can do enough damage in an afternoon to throw off the entire chemical balance." Still, Holman suggests, it's a very practical solution for hyper-efficient cultivation of food. "We know it's scalable, we're just experimenting to see what works."