Popular Mechanics: When pre-Columbian natives in the Amazon Basin first began to use biochar–a fine-grained, carbon-rich type of charcoal made from burning bone fragments and other food remains–some 7,000 years ago, they knew that it helped their crops grow. But they didn’t realize that this charred biomass was extraordinarily good at absorbing and storing carbon dioxide, and that the process that made it released chemicals that could be used as fuel. (At the time, chemistry was still a few thousand years …