Last month, Doug Wiens, PhD, professor of earth and planetary science at Washington University in St. Louis, and two WUSTL students were cruising the tropical waters of the western Pacific above the Mariana trench aboard the research vessel Thomas G. Thompson. The trench（渠沟） is a subduction zone, where the ancient, cold and dense Pacific plate slides beneath the younger, lighter high-riding Mariana Plate, the leading edge of the Pacific Plate sinking deep into Earth's mantle as the plates slowly converge（聚合） .

Taking turns with his shipmates, Wiens swung bright-yellow ocean bottom seismometers（地震仪） and hydrophones off the fantail, and lowered them gently to the water's surface, as the ship laid out a matrix of instruments for a seismic survey on the trench.

The survey, which Wiens leads together with Daniel Lizarralde, PhD, of the Woods Hole Oceanographic Institution, will follow the water chemically bound to the down-diving Pacific Plate or trapped in deep faults that open in the plate as it bends. The work is funded by the National Science Foundation.

Scientists have only recently begun to study the subsurface water cycle, which promises to be as important as the more familiar surface water cycle to the character of the planet.

Hydration reactions along the subducting plate are thought to carry water deep into Earth, and dehydration（脱水） reactions at greater depths release fluids into the overlying（上覆盖的） mantle that promote melting and volcanism.

The water also plays a role in the strong earthquakes characteristic of subduction zones. Hydrated rock and water under high pressure are thought to lubricate the boundary between the plates and to permit sudden slippage.