On dry land, most organisms are confined to the surface, or at most to altitudes of a hundred meters—the height of the tallest trees. In the oceans, though, living space has both vertical and horizontal dimensions: with an average depth of 3800 meters, the oceans offer 99% of the space on Earth where life can develop. And the deep sea, which has been immersed in total darkness since the dawn of time, occupies 85% of ocean space, forming the planet’s largest habitat. Yet these depths abound with mystery. The deep sea is mostly uncharted—only about 5 percent of the seafloor has been mapped with any reasonable degree of detail—and we know very little about the creatures that call it home. Current estimates about the number of species yet to be found vary between ten and thirty million. The deep sea no longer has anything to prove; it is without doubt Earth’s largest reservoir of life.