Gypsum sand dunes, White Sands National Park, New Mexico, USA

Sands of time

At first glance, you might mistake these dunes for massive snowdrifts. Although you can sledge down them, the tiny crystals that make up White Sands National Monument in New Mexico, USA are not ice but gypsum, a soft mineral often used to make plaster and chalk. The pearly sands compose the largest gypsum dune field in the world but cover just a fraction of southern New Mexico's Tularosa Basin.

This vast desert valley, bigger than some US states at 6,500 square miles, is mostly occupied by White Sands Missile Range. The active military installation surrounds the National Monument and includes the Trinity site, where the first atomic bomb was detonated in 1945. The White Sands region is also defined by echoes from our very distant past: scientists have used radar technology to discover prehistoric human, mammoth and giant sloth footprints buried long ago beneath the shifting sands.