A shimmering travertine deposit forms a limestone terrace around a mineral spring at Mammoth Hot Springs in Yellowstone National Park.

This formation, called Minerva Terrace, formed as mineral-laden water bubbled over and evaporated, leaving a sparkling white calcium-carbonate crust. These deposits, which can accumulate at up to a foot (30 centimeters) per year, create a spectacular and constantly changing landscape.

(Photo shot on assignment for, but not published in, “The Planets: Between Fire and Ice,” January 1985, National Geographic magazine)