In the summer of 2002, scientist and entrepreneur Danny Day sent a lab assistant to retrieve some charcoal from behind Day’s lab in Blakely, Georgia. At the time, he was researching how to turn peanut shells into hydrogen for the U.S. Department of Energy. He regularly used charcoal to preheat the reactor. When his assistant returned, he came bearing strange news: Plants had taken root in the bed of charcoal-weeds, grass, turnips as big as baseballs, enough to fill four garbage bags.

How had turnips sprouted from a pile of charcoal? Day wondered.

Charcoal is easy to make. Take biomass like wood, leaves and grass clippings, and burn it in an oxygen-free setting until all that remains is a bit of ash and a bunch of carbon. Typically, the carbon is released into the atmosphere when charcoal is burned. But if you buried the charcoal instead, the carbon would remain safely captured. And if charcoal helps turnips grow as big as baseballs, there might be a very good reason to bury it.

Such was Day’s thinking as he rushed behind his lab with a sample bag, a -microscope and a digital camera. In the eight years since that moment, ‘biochar