When you thrust a shovel into the soil or tear off a piece of coral, you are, godlike, cutting through an entire world. You have crossed a hidden frontier known to very few. Immediately close at hand, around and beneath our feet, lies the least explored part of the planet's surface. It is also the most vital place on Earth for human existence.
In any habitat, on the ground, in the forest canopy, or in the water, your eye is first caught by the big animals—birds, mammals, fish, butterflies. But gradually the smaller inhabitants, far more numerous, begin to eclipse them. There are the insect myriads creeping and buzzing among the weeds, the worms and unnameable creatures that squirm or scuttle for cover when you turn garden soil for planting. There are those annoying ants that swarm out when their nest is accidentally cut open and the pesky beetle grubs exposed at yellowed grass roots. When you flip a rock over, there are even more: You see spiderlings and sundry pale unknowns of diverse form slinking through mats of fungus strands. Tiny beetles hide from the sudden light, and pill bugs curl their bodies into defensive balls. Centipedes and millipedes, the armored snakes of their size class, squeeze into the nearest crevices and wormholes.
It may seem that the whole icky lot of them, and the miniature realms they inhabit, are unrelated to human concerns. But scientists have found the exact opposite to be true. Together with the bacteria and other invisible microorganisms swimming and settled around the mineral grains of the soil, the ground dwellers are the heart of life on Earth.