Leonardo da Vinci once mused that "we know more about the movement of celestial bodies than about the soil underfoot," an observation that is as apt today as it was five hundred years ago. The biological world under our toes is often unexplored and unappreciated, yet it teems with life. In one square meter of earth, there lives trillions of bacteria, millions of nematodes, hundreds of thousands of mites, thousands of insects and worms, and hundreds...