Chatters is best known for his work on Kennewick Man, an ancient skeleton found in Kennewick, Washington, in 1996, whose origins were debated, because his skull was markedly different from those of modern Native Americans.

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The newfound skeleton was named "Naia," after Greek water spirits known as naiads.

The bones are the nearly intact remains of a small, delicately built teenage girl who stood about 4 feet 10 inches (149 centimeters) tall and was about 15 or 16 years old at the time of her death, based on the development of her skeleton and teeth.

This underwater chamber is reachable only by divers in the Sac Actun cave system, a web of flooded tunnels beneath the jungles of Mexico's Eastern Yucat√°n Peninsula.

"Hoyo Negro is a more than 100-foot-deep (30 meters), bell-shaped, water-filled void about the size of a professional basketball arena deep inside a drowned cave system," Chatters said. First they must climb down a 30-foot (9 m) ladder in a nearby sinkhole.

Based on direct radiocarbon dating of tooth enamel and indirect uranium-thorium dating of flowerlike crystalline deposits on Naia's bones, the researchers suggest her remains are 12,000 to 13,000 years old.

This hinted that she could help reveal a long-standing controversy regarding the mysterious relationship between the earliest Americans and modern Native Americans.

"We had no idea what we might find when we initially entered the cave, which is the allure of cave diving," said study author Alberto Nava of Bay Area Underwater Explorers in Berkeley, California.

"The moment we entered the site, we knew it was an incredible place.

[Human Origins: How Hominids Evolved (Infographic)] "Modern Native Americans closely resemble people of China, Korea, and Japan, but the oldest American skeletons do not," Chatters said.