Deep biosphere: a hidden, watery world exists beneath the oceans' floors.

Beyond the limits of most of the latest ocean observatories lies yet another frontier: a watery realm below the seafloor. Earlier this year, researchers made the first expedition to study life in this deep biosphere. They found microbes in cores drilled 420 meters into the seafloor off Peru at depths between 150 and 5,300 meters. These microbes might represent as much as two-thirds of Earth's entire bacterial biomass. Scientists propose that similar life might exist below oceans on other planets and some moons.

Before discovering life in the deep biosphere, marine geologists knew from seismic and drilling observations that the seafloor is porous and filled with seawater. Geologists also knew that this buried seawater, or subseafloor groundwater, moves heat from beneath the seafloor to the oceans. They wanted to learn how fast this groundwater moves, so they installed meter-wide observatories on top of boreholes drilled into the subseafloor. The boreholes are about 25 centimeters in diameter and 100 to 1,000 m deep. Sensors dangling into the holes continuously record the groundwater pressure and temperature.

The observatories are called circulation obviation retrofit kits, or CORKs. Soon after their 1991 debut, the observatories revealed that cooling seawater can move quickly through several kilometers of subseafloor. The scientists reached this conclusion when they measured similar temperatures in rock formations that they had expected to hold different amounts of heat.

"Fluids are whistling around clown in this subseafloor ocean," says Earl E. Davis, a geophysicist at the Geological Survey of Canada in Sidney, British Columbia.

Researchers were surprised to find that their instruments also recorded far more subtle and transient events, such as the tides above them. "Those [events] actually propagate into the subseafloor, and we can watch those," says Davis. As sea levels or even atmospheric pressures rise and fall, the subseafloor changes shape, he says.

Scientists are also using CORKs to watch what happens to the subseafloor during earthquakes and movements of tectonic plates in the seafloor. Because this environment is completely saturated with water, CORK measurements are free from water table fluctuations and precipitation effects that distort borehole measurements on land.

"We're able to look at some of the processes--like earthquake rupture processes or strain buildup before, during, and immediately after an earthquake--that can't be [observed] otherwise," explains Davis. He and his colleagues reported these findings in the October 2001 Journal of Geophysical Research.

Last month, the ship JOIDES Resolution installed two more CORKS in a seismically active area off Costa Rica--bringing the worldwide total to 19. According to Keir Becker, a geophysicist at the University of Miami, scientists aboard the ship also retrieved data from the deepest borehole ever instrumented. The hole penetrates 2,111 m into the seafloor beneath 3,500 m of ocean.--C.M.