Nine miles from the volcano the Maasai call the “Mountain of God,” researchers have cataloged a spectacularly rare find: an enormous set of well-preserved human footprints left in the mud between 5,000 and 19,000 years ago.

The more than 400 footprints cover an area slightly larger than a tennis court, crisscrossing the dark gray mudflat of Engare Sero, on the southern shore of Tanzania’s Lake Natron. No other site in Africa has as many ancient Homo sapiens footprints—making it a treasure trove for scientists trying to tell the story of humankind’s earliest days.

Some of the tracks seem to show people jogging through the muck, keeping upwards of a 12-minute-mile pace. Other prints imply a person with a slightly strange, possibly