IT’S crawling with life down there. A remote expedition to the deepest layer of the Earth’s oceanic crust has revealed a new ecosystem living over a kilometre beneath our feet. It is the first time that life has been found in the crust’s deepest layer, and an analysis of the new biosphere suggests life could exist lower still.

On a hypothetical journey to the centre of the Earth starting at the sea floor, you would travel through sediment, a layer of basalt, and then hit the gabbroic layer, which lies directly above the mantle. Drilling expeditions have reached this layer before, but as the basalt is difficult to pierce it happens rarely.

To facilitate the task, the Integrated Ocean Drilling Programme set its sights on the Atlantis Massif. Tectonic activity beneath this submerged mountain in the central Atlantic Ocean has pushed the gabbroic layer within 70 metres of the sea floor, making it easier to reach (see diagram). A team led by Stephen Giovannoni of Oregon State University in Corvallis drilled down to 1391 metres, where temperatures reach 102 °C.

There, they found communities of bacteria that were sparse but widespread. The type of bacteria they found came as a surprise to Giovannoni, who has previously found micro-organisms living in the basalt layer. “We expected to find similar organisms in the deeper layer,” he says. “But actually it was very different.”


One key difference was that archaea were absent in the gabbroic layer. Also, genetic analysis revealed that unlike their upstairs neighbours, many of the gabbroic bugs had evolved to feed off hydrocarbons like methane and benzene. This is similar to the bacteria found in oil reservoirs and contaminated soil, which could mean that the bacteria migrated down from shallower regions rather than evolving inside the crust, the team say (PLoS ONE, DOI: 10.1371/journal.pone.0015399).

“This deep biosphere is a very important discovery,” says Rolf Pedersen of the University of Bergen, Norway. He points out that the reactions that produce oil and gas abiotically inside the crust could happen in the mantle, meaning life may be thriving deeper yet.