Gypsum sand dunes, White Sands National Park, New Mexico

Sands of time

At a quick glance, you might mistake these dunes for massive snowdrifts. Although they do make for great sledding, the tiny crystals that make up White Sands National Park in New Mexico, 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—more extensive than some U.S. states at 16,800 square kilometres—is mostly occupied by White Sands Missile Range. The active military installation—the US's largest by area—surrounds the national park and includes the Trinity site, where the first atomic bomb was detonated in 1945. That event altered the course of humanity's future, but the White Sands region is also defined by echoes from our very distant past: recently, scientists have used radar technology to zero in on prehistoric human, mammoth, and giant sloth footprints buried long ago beneath the shifting sands.