Spelunkers rappel 300 feet (90 meters) into the Greenland ice sheet. Glacier caves such as these form when seasonal meltwater or geothermal vents cut fissures and channels through an ice sheet. Because they are made of ice, glacier caves can be quite unstable and present unique challenges to spelunkers

A waterfall cascades down the sheer face of the Grand Canyon in Arizona. In addition to the mighty Colorado River running through the canyon, water—a vital resource in the arid Southwest—exists in the form of springs, streams, and seeps.

With Mexico on the left and the United States on the right, the Rio Grande forges a clear barrier between the two countries. On the U.S. side the river winds through the Santa Elena Canyon in Texas' Big Bend National Park. Towering 1,500-foot (460-meter) cliffs of solid limestone mark the canyon.