The pair had been diving for years near Sinibaldi's property on the Steinhatchee River. Combing the riverbed for fossils isn't easy; the water is full of tannins, which significantly reduces visibility. "It's like diving in coffee," Sinibaldi said.

They were on their usual fossil hunting trip in June of 2022 and weren't having much luck. They were preparing to move on when Branin looked down and happened to see horse teeth. As they continued looking, they uncovered a hoof core, and then a tapir skull.

Their good feeling swelled to disbelief as the finds continue to rack up—many in pristine condition. "It wasn't just quantity, it was quality," Sinibaldi said. "We knew we had an important site, but we didn't know how important."

The Steinhatchee River likely followed a different course when the fossils were preserved, but as it meandered over the following millennia, the river edged closer to the sinkhole until, very recently, it eroded into the former pit and rinsed the fossils, leaving them exposed along the bed of the river.

He and Branin shared their findings with the Florida Museum of Natural History, where paleontologists determined they were preserved during an obscure period of the Pleistocene ice ages called the middle Irvingtonian.

"The fossil record everywhere, not just in Florida, is lacking the interval that the site is from—the middle Irvingtonian North American land mammal age," said Rachel Narducci, vertebrate paleontology collections manager at the Florida Museum and co-author of a new detailed study of the site. The study is published in the journal Fossil Studies.