English: This map shows changes in ocean bottom pressure measured by NASA's Gravity Recovery and Climate Experiment (GRACE).

Red shows where pressure varies by large amounts, blue where it changes very little.Just as knowing atmospheric pressure allows meteorologistss to predict winds and weather patterns, measurements of ocean bottom pressure provide oceanographers with fundamental information about currents and global circulation. They also hold clues to questions about sea level and climate.

The pressure at the bottom of the ocean is determined by the amount of mass above it. Launched in 2002, the twin Grace satellites map Earth's gravity field from orbit 500 kilometers (310 miles) above the surface. They respond to how mass is distributed in the Earth and on Earth's surface -- the greater the mass in a given area, the stronger the pull of gravity from that area.