Based on the skull's location, the team believes the remains ended up there about 10,000 years ago—just before the then dry cave was inundated as sea level began rising. If confirmed, that age would make the skull one of the oldest known remains of an early American, or Paleo-Indian.
Though the skull was found alongside bones of a mastodon and other prehistoric animals in 2007, news of the find was released only late last month, to allow time to properly document the site, train the divers in archaeological practices, and coordinate with authorities.
The divers had previously seen Ice Age animal remains in surrounding caves, but the human skull was a surprise. "That's one of the beauties of exploration—you never know what you're going to find," diver Alberto Nava said.
Now comes the tricky part. "We want to figure out the story of Hoyo Negro and how the human and animal remains got there," said Nave, of the Projecto Espeleológico de Tulum (PET) organization and Global Underwater Explorers (GUE).
(Read the account of a National Geographic archaeologist involved with the project: "Skull in Underwater Cave May Be Earliest Trace of First Americans.")
Note: National Geographic magazine partially funded archaeological training for the divers on this expedition. Both the magazine and National Geographic News are parts of the National Geographic Society.