Far down beneath accumulated layers of sediment, the floor of the ocean is made of basaltic rock. This rock was formed from molten lava that oozed up from deep within the planet at the boundaries of Earth’s tectonic plates. It is difficult to reach, and mostly unexplored.

In 2020 a team of Japanese scientists reported their surprising finding that sea floor rock contains habitats rich in microbial life. The researchers used samples recovered from four hundred feet beneath the bed of the Pacific Ocean by the Integrated Ocean Drilling Program, a collaborative international project. The rock sampled ranged in age from thirteen to one hundred million years old.

The researchers found that this sea floor basalt was laced with tiny cracks, sometimes less than a millimeter wide. These cracks were filled with clay minerals, which are noted for their ability to concentrate nutrients useful to microbes. The investigators sliced the rock into thin sections, and applied a glowing fluorescent stain that labels DNA. Cracks in the rocks glowed brightly with the label, indicating the presence of abundant microbial life.

The cracks contained about ten billion bacterial cells per cubic millimeter, which is as dense as the concentration of bacteria in the human gut. By contrast, the mud sediment lying above the rock was sparsely populated, containing only one hundred cells per cubic centimeter.

The researchers believe that their findings may be important to the ongoing search for microbial life on Mars. Like Earth’s seafloor, the Martian subsurface is rich in basaltic rock. Cracks in this rock might offer clement habitats for potential microbial life sheltered from the harsh conditions of the Martian surface.