Microbiologist James Holden:

"Evidence has built over the past 20 years that there's an incredible amount of biomass in Earth's subsurface, in the crust and marine sediments, perhaps as much as all the plants and animals on the surface. We're interested in the microbes in the deep rock, and the best place to study them is at hydrothermal vents at undersea volcanoes. Warm water flows bring the nutrient and energy sources they need."
Some scientists estimate that, in terms of mass, a third of earth's organisms live in the ground, in rocks, in the cracks.