More than 10,000 years ago, a band of hunter-gatherers chased a group of giant ground sloths along the shores of an ancient lake, and their footprints still preserve the story. West of the sparkling white gypsum dunes of White Sands National Monument, at a site called Alkali Flat, layers of mud and sand left behind by a long-vanished lake hold more than 100 human and giant ground sloth tracks—rare evidence of Pleistocene hunters stalking, and sometimes cornering, giant sloths.

Most of them are so-called “ghost tracks,” visible only under very specific conditions.

“Most of the time they are invisible. There is a lot of salt in ground, and when it rains, the salt dissolves. And, crucially, as it dries out, the fill dries at a different rate and the difference between the fill (footprint) and the surrounding sediment makes the track visible for a brief time while it dries out,” explained paleoecologist Sally Reynolds of the UK’s Bournemouth University, a co-author on the paper. The team, led by the US National Parks Service’s David Bustos, used aerial photography to spot the tracks and then selected a few groups for careful excavation and study.