The footprints look ordinary enough, those of young kids and teenagers walking near what was once a lake. But a new study in Science adds a remarkable twist—scientists say the prints were made about 23,000 years ago in what is now White Sands National Park in New Mexico. The assertion challenges a long-held belief that the first humans came to North America only about 13,000 years ago, when glaciers retreated and made their path possible, reports the New York Times. Previous research has raised doubts about the latter timeline, but proof has been elusive. Which is why National Geographic describes this new study as a "bombshell" and quotes an archaeologist with Mexico's Autonomous University—uninvolved in the study—who says "a discovery like this is very close to finding the Holy Grail."