MEXICO CITY - A new investigation reveals the skeleton of a girl discovered in a Mexican cave is at least 12,000-years-old. The discovery is shedding new light on how the Americas were settled.

Divers make their way through an underground system of flooded caverns, or Cenotes, near the Mexican resort of Tulum. The conditions are challenging. They need to swim 1,200 meters before reaching a large cavern known as the black hole.

It was in this cavern in 2007 where one diver discovered a human skeleton now believed to be at least 12,000-years-old. She was named Naia, thought to be around 16 when she died, and could not be removed from the site.

"There are challenges but there are also benefits. This site was never touched so it's pristine, it's fantastic. On the other hand it's very difficult: None of the scientists have gone inside the cave. It's like working remote sensing," said Pilar Lunar, head of Underwater Archaeology at IHAH.

Archaeologists formed a unique link with expert divers who also discovered 26 mammals, including the now-extinct sabre tooth tiger shown here. Photographs were closely analyzed, but the priority was to trace the human remains.

"We have to get a molar out of the human skull of Naia and that one was secured, brought out of the Cenote and was taken samples out of it," said Joaquin Arroyo, head of Laboratories, INAH.

After independent carbon dating, they discovered a link to Siberia in Russia, meaning Naia's descendants crossed over the Bering Strait when it was a land mass, and into what's now the American continent.

"Naia is the oldest one, the most complete skeleton, genetically complete as well, that has allowed us to prove that and that's fantastic," said Pilar.

Mexicans CNN spoke to seemed to welcome the news.

"We are not so different that we think sometimes. Not more Russian, but more connected," said one man.

The government is now stressing the need to preserve the thousands of other Cenotes in Mexico in the hope they too may yield more historic treasure.