Last week, explorers with the Great Maya Aquifer Project discovered a connection between two large underwater caverns on the Yucatan Peninsula. When combined, the two systems create a 215-mile-long underground labyrinth—the largest flooded cavern on Earth, reports National Geographic.

While the cave itself is an interesting geologic formation, the cave system is also full of pre-Hispanic archaeological sites from the ancient Maya as well as unknown plant and animal species. “This immense cave represents the most important submerged archaeological site in the world, as it has more than a hundred archaeological contexts,” says Guillermo de Anda, director of the project, according to a translated press release. “Along this system, we had documented evidence of the first settlers of America, as well as extinct fauna and, of course, Maya culture." In fact, in 2014, divers found the oldest human skeleton discovered in the New World while exploring one of the segments of this submerged cavern, Sac Actun.

As National Geographic reports, the discovery was made after the project's divers began a new phase of exploring the Sac Actun system and another known as Dos Ojos last March, mapping new tunnels and underground lakes, known as cenotes. They were also looking for a connection between the two. After months of exploration, they finally found it: a subsurface connection near the city of Tulum, Reuters reports. According to cave-naming protocols, the larger system will absorb the smaller system and the whole complex will be known as Sac Actun.

“This is an effort of more than 20 years of traveling hundreds of kilometers of caves submerged in [the Mexican state of ] Quintana Roo mainly, of which I devoted 14 to explore this monstrous Sac Actun System,” the project's director of exploration Robert Schmittner says in the press release. “Now, everyone's job is to keep it [up]."

Before this discovery, the largest underwater cave system was the nearby, 168-mile Ox Bel Ha system, followed by Sac Actun, Koal Baal and Dos Ojos systems. Researchers believe that all of these systems are likely connected, making up the Great Maya Aquifer. As National Geographic reports, the next phase of exploration will be trying to identify the connections between Sac Actun and these other sections.

During their explorations, the researchers also found another new system north of Sac Actun that is 65 feet deep and contains 11 miles of caverns. For now, the system stands alone, but as National Geographic reports, there's still a possibility the team may find a connection.

Thomas Iliffe, a marine cave researcher at Texas A&M, tells Sydney Pereira at Newsweek that mapping and exploring such huge underground systems is painstaking work. The winding carverns have various levels, endlessly branch and have plenty of dead ends. Getting lost in such a place can be deadly. “These really are maze-like systems,” he says.

For the ancient Maya, certain caves were considered sacred pilgrimage sites where priests could go to communicate with the gods. They were also sites of sacrifice. One cavern, the Midnight Terror Cave in Belize contains almost 10,000 bones of children—all younger than 14—who were sacrificed to Chaac, the god of rain, lightning and water.

One thing is for sure: There is still much more to learn about the region's underground caverns. Just last November, researchers from the Great Maya Aquifer project found evidence of a blocked passageway below the main temple at the world heritage site Chichén Itzá. Researchers speculate it may lead to a sinkhole beneath the temple, which could help further reveal the ancient Maya culture's connection to the region's vast underworld.