Nine sets of human skeletal remains have been found in the underwater caves, whose passages can be barely big enough to squeeze through.

The discovery of remains of human-set fires, stacked mining debris, simple stone tools, navigational aids and digging sites suggest humans went into the caves around 10,000 to 12,000 years ago, seeking iron-rich red ocher, which early peoples in the Americas prized for decoration and rituals.

The early miners apparently brought torches or firewood to light their work, and broke off pieces of stalagmites to pound out the ocher.

"While Naia added to the understanding of the ancestry, growth and development of these early Americans, little was known about why she and her contemporaries took the risk to enter the maze of caves," wrote researchers from the Research Center for the Aquifer System of Quintana Roo, known as CINDAQ for its initials in Spanish.

Roberto Junco Sánchez, the head of underwater archaeology for Mexico's National Institute of Anthropology and History, said the discovery means the caves were altered by humans at an early date.

"Now we know that ancient humans did not risk entering this maze of caves just to get water or flee from predators, but that they also entered them to mine," Junco Sanchez said.

However, James Chatters, forensic anthropologist, archaeologist, and paleontologist with Applied Paleoscience, a consulting firm in Bothell, Washington, noted that none of the pre-Maya human remains in the caves were found directly in the mining areas.

"It gives them a reason" to go into the caves, Pelton said, adding: "Considering the massive scale of this mining, it's the first thing I would go for.".

The caves provide a well-preserved environment and are where one of the oldest sets of human remains found in the Americas, a young woman nicknamed "Naia," was discovered in 2007.