Sacred Mayan underwater tunnels in Mexico's Yucatan are being rediscovered for the first time in thousands of years, scientists have revealed.
An extensive network of limestone caves housing Mayan artefacts is just now being explored by scientists from Mexico's National Anthropology and History Institute (INAH).
What INAH researchers have found is a remarkable underwater system, measuring more than 340km in length, which connects two previously known cenotes – Spanish for 'giant sinkhole aquifers' – near Tulum, in the state of Quintana Roo.
Scuba diving in the immense tunnels lined with stalactites, researchers have found highly preserved ceramic vases from Mayan society which would have been used in funerals and during ritual sacrifices.
They have also found intact human skulls, along with bones from elephants, giant sloths, bears, tigers and extinct species of horses, along with now-extinct plants. Investigators say the artefacts are well preserved because of the caves' inaccessibility.
Cenotes, known as dzonoot in Mayan, were a central part of the Mayan society and religion that flourished in what is now Mexico's Yucatan, along with the Central American countries of Guatemala, Honduras and El Salvador.
"This immense cave represents the most important submerged archaeological site in the world," says Guillermo de Anda, director of the INAH study, noting that Mayans believed cenotes were the third level of the universe, after the heavens and Earth.
Cenotes are "a very powerful, magical space where the supernatural reigned, where gods and goddesses lived, where the good and bad co-existed, where man came from," says Anda. "It's a tunnel of time that transports you to a place 10,000 to 12,000 years ago."
Efforts to locate the tunnel began more than 14 years ago, headed by lead diver Robert Schmittner, who had spent 20 years trying to find a direct connection between the Sac Actun cenote in the south of the state and Dos Ojos cenote toward the north.
"We came really close a few times," Schmittner tells El Pais newspaper. "On a couple of occasions, we were a meter from making a connection between the two large cave systems."
Schmittner and INAH researchers began their focused investigation 10 months ago and only recently discovered the underwater route connecting the two cenotes.
"It was like trying to follow the veins within a body," says the diver. "It was a labyrinth of paths that sometimes came together and sometimes separated. We had to be very careful."
At certain points, the tunnel shrinks from 120 meters in width to just one meter.
In Quintana Roo, there are 1,400km of known underwater tunnels. Investigators are now trying to determine the composition of the water found within the labyrinth.
The research team has yet to ascertain exactly how the ancient Mayan artefacts came to rest in the cenotes for thousands of years.
This isn't the first time human remains have been found in Tulum cenotes, but it is the largest site yet identified. In 2010, researchers resurfaced the 10,000-year-old skeleton of a boy: some of the oldest human bones found in the Americas to date.