We don't know what we're standing on. The roughly 8,000 miles of dirt, solid rock, water, molten rock, and God-knows-what-else under our feet are for the most part as mysterious to us as the moons of Jupiter—and the visible world we live on, the world recorded science has spent four thousand years studying, is only its epidermis. It isn't completely unreasonable to assume that most of this world is lifeless, that the living species of the world merely live on it, like moss on a stone—but what if there's life hiding under the surface?
On Wednesday, a team of scientists published an article suggesting the existence of a "hydrous mantle transition zone" larger than every ocean and sea on Earth combined. (If you don't have a Nature subscription and want more than the abstract, Scientific American explains the article in glorious detail here.) The discovery of organisms surviving without sunlight in hydrothermal vents introduced us to the concept of chemosynthesis, and suggested that life already exists in pockets of the upper mantle. But now that we know we're dealing with more than mere pockets of water, the possibility of an entire ecosystem thriving in a warm superocean inside the Earth's mantle is a very realistic one.
Over the next ten years, the Integrated Ocean Drilling Program's Chikyú vessel may tell us more about the composition of the mantle by drilling through the ocean floor to get to it:
But right now, the deepest into the Earth we can get is a little less than six miles (about 0.07% of the Earth's diameter)—and that's on land, where we're in no danger of cracking into the mantle. When we're in a better position to really take a look at what's underneath us, we may be surprised by what we find.