To understand what the footprints preserve, you have to understand the landscape they were pressed into — because that landscape is gone, and what replaced it is almost the opposite of what was there.

White Sands National Park sits in the Tularosa Basin of southern New Mexico, a broad valley between the San Andres Mountains to the west and the Sacramento Mountains to the east.

Today, the basin contains the largest gypsum dune field in the world — approximately 275 square miles of white gypsum sand that shifts and flows in the desert wind, covering the basin floor in a landscape that looks, from above, like a frozen white sea.

It is one of the most striking landscapes in North America, and one of the most inhospitable.

Rainfall is minimal.

Temperatures reach extremes.

The white gypsum reflects sunlight with punishing intensity.

Water, apart from occasional monsoon rains, is absent.

Twenty-three thousand years ago, none of that was true.

The Tularosa Basin in the Last Glacial Maximum was a fundamentally different environment.

The glaciers that covered the northern part of the continent altered weather patterns across the entire hemisphere.

The Southwest was not glaciated — the ice sheets never reached this far south — but the climate was significantly cooler and wetter than it is today.

Rainfall was higher.

Evaporation was lower.