Nearly a mile underground, there's an entire ecosystem carving out a rather alien existence in the deepest layer of Earth's crust. These bacteria are totally unlike their brethren nearer the surface, and there might be even stranger bacteria further down.
Earth's crust only covers the top 1% of the planet's outer volume. The oceanic crust, which unsurprisingly is the part of the crust underneath the seas, is about 6 to 10 kilometers thick, and it has three layers of its own: a thin sediment, a tough layer of basalt, and then lots of gabbro, another rock very similar to basalt. Because the basalt layer is so hard to drill through, we still don't have a good idea of just what's in the gabbroic layer.
That's why Oregon State researchers focused on a part of the ocean where tectonic activity had pushed the gabbro up past the basalt to within 70 meters of the ocean bed, making it much easier to access. They drilled down 1,391 meters into the gabbroic layer of the crust. They discovered a world teeming with multiple little pockets of thriving bacteria.
Previous research has found micro-organisms living in the basalt layer, but these gabbroic bacteria are very different from their relatives in the layer above them. For a start, the creatures in the gabbro are exclusively bacteria, with no archaea to be found.
The gabbroic bacteria also rely on totally different food sources. They subsist on hydrocarbons such as methane or benzene, much like the bacteria found in oil slicks or contaminated soil. The researchers suspect the bacteria evolved nearer the surface and then migrated down to their present depths.
This gabbroic ecosystem is the deepest biosphere yet discovered, but it may not be the end of the story. The same reactions that produce hydrocarbons in the crust could also occur in the mantle, the next, much bigger layer of the Earth below. It could be a while until we can drill down far enough to find out, but there could quite possibly be bacteria in the mantle. It's not quite living dinosaurs and subterranean oceans, but it appears a real journey to the center of the Earth would be far livelier than we ever suspected.