Alejandro Alvarez's eyes widened against the dark underwater void that would become known as the Black Hole on Mexico's Yucatan Peninsula.
His flashlight shined on ancient bones from extinct species, and eventually he would discover the hemisphere's oldest, most complete skeleton, a find that may transform the way we think about the development of American man.
"What in the world is this?" Alvarez recalls thinking. He and two diving buddies with him knew that they had stumbled across something special.
"We immediately realized the importance," Alvarez, now 52 and still diving, said in an interview. "It was very exciting."
The discovery of the 12,000-year-old skeleton of a teenage girl occurred seven years ago but wasn't announced until this month, after additional, sometimes-risky exploration and detailed scientific investigation.
Published first in the American magazine Science, then elaborated upon by Mexican scientific officials, the find has provided immeasurable evidence on the origins of the first Native Americans.
"This reveals a huge world," Teresa Franco, director of Mexico's National Institute of Anthropology and History, which took over study of the skeleton, said in a news briefing.
Yet such a remarkable discovery started out as nothing but a hobby for Alvarez and his small group of amateur spelunkers.
Alvarez, a civil engineer by training, had been diving much of his adult life. More recently he took up exploration of some of the estimated 6,000 water-filled caves that dot the Yucatan.
And so it was that on May 10, 2007 — he remembers the exact date because it is Mother's Day in Mexico — Alvarez and two fellow explorers, Alberto Nava and Franco Antonelli, swam into La Virgen, in Quintana Roo state, one of the caves known locally as cenotes.
They went deeper and deeper, down 10 yards and then along a tunnel more than half a mile long.
"It was immensely dark," Alvarez recalled. Suddenly the floor disappeared. This was the Hoyo Negro, the "black hole."
"We had to follow along the wall," Alvarez said. "I happened to be in the front. Then I could see something floating."
It turned out to be the leg bone of a gomphothere, an ancient elephant-like creature, one of dozens of bones they would find. "We stopped there, wide-eyed," Alvarez said.
They returned the next day and for many weeks to follow, this time with propulsion equipment to move more easily into the depths and hauling nine breathing tanks so they could stay below longer.
Soon they made their most celebrated discovery: They spied the girl's skull, resting on a ledge, propped up by her arm and bearing a full set of teeth.
"We knew this was a very important find, but for two years we could not figure out just what to do with it," Alvarez said from his home in Tulum.
Finally, they made videos of the skull, skeleton and the rest of the watery graveyard and reported to the National Institute.
Since then, late 2009, a team of U.S. and Canadian investigators has been working with the Mexicans to explore and document the Hoyo Negro site.
The age of the skeleton was determined through DNA testing of rib bones and a tooth, along with testing of seeds and bat droppings found in the Hoyo Negro, officials said.
Scientists believe the girl fell into the deep hole before water filled the caves. As the ice age ebbed and prehistoric glaciers melted, she and animals, such as saber-toothed tigers, were covered in water, sealed off from potential marauders or grave robbers.
The researchers nicknamed her Naia, for the ancient Greek water nymphs. The scientists believe she will help shed light on the theory held by most, but not all, scholars that the New World was settled by people who came from Siberia, crossing the Bering Strait.
"This is the oldest, most intact skeleton ever found" in the Americas, said Pilar Luna Erreguerena, head of subaquatic archaeology at the National Institute.
"And it was found in a perfect context, virtually untouched, like a time capsule."
Some studies have dated prehistoric man in the Americas to at least 40,000 years ago. Acquiring a such a skeleton allows scientists to learn more about early humans' features, evolution and how they lived, experts said.
Alvarez, who runs a spelunking company that caters to tourists in Tulum, says he and his fellow divers made no money from their discovery, although authorities began providing the equipment. Still, he and his companions remain involved in and excited about the project.