The Southern Patagonian Ice Field, which straddles Chile and Argentina, feeds glaciers, lakes, and rivers that ultimately flow into the Pacific and Atlantic Oceans. Scientists now can remotely monitor sea ice, snow depths, and soil moisture there and elsewhere by using GPS signals reflected from Earth’s surface. To read about how GPS antennas sense the hydrological cycle, turn to Clara Chew’s article on page 42 . (Landsat 8 image, courtesy of NASA’s Earth Observatory/Jesse Allen, using data from the US Geological Survey.)