Archaeologists have known about the sinkhole in the Aucilla River, south of Tallahassee, FL, for years. Because it is so dark, there’s never been dives in there until Jessi Halligan, a professor at Florida State University, decided to conduct research.

Her project brought unexpected results: her diving expedition found mastodon tusks that had long grooves left by human tools — and those were found nearby as well. The discovery strongly suggests that humans inhabited the Florida area much earlier than had been thought initially, dating back 14,500 years.