The “cave of the jaguar god,” an underground treasure trove of ancient Mayan artifacts, has been explored for the first time since it was deliberately walled off more than 50 years ago.

Located under the ancient city of Chichén Itzá, Mexico, the cave system is packed with hundreds of items dating back more than 1,000 years, such as decorative plates, grinding stones, incense holders, and figures of Balamkú, its namesake jaguar god.

"Balamkú will help rewrite the story of Chichen Itzá,” said Guillermo de Anda, co-director of Great Maya Aquifer Project (GAM) and an archeologist at the National Institute of Anthropology and History (INAH), at a press conference in Mexico City on Monday. The hundreds of archaeological artifacts “are in an extraordinary state of preservation,” he added.

The cave was originally discovered by local farmers in 1966, who alerted an archeologist named Víctor Segovia Pinto. For some unknown reason, Segovia decided not to explore the cave in much detail and instead blocked the entrance with rocks.

James Brady, co-director of GAM and a professor of anthropology at California State University in Los Angeles, has been puzzling over Segovia’s decision to seal the cave.

“I’m trying figure it out,” he told Motherboard in a phone call. “[Segovia] may have already been committed to other projects and knew that he did not have the time or resources to do it. That may have been why he told them to close it up and leave it.”

Segovia, who has since died, did file a small internal report about the discovery with INAH. But the cave was otherwise overlooked for five decades until de Anda was tipped off to it by Luis Un, a local resident.

Un, 68, was one of the people who first explored Balamkú in 1966, when he was still a teenager. Last year, he led de Anda and his colleagues to the cave entrance, where the team quickly recognized the pristine condition of the artifacts.

“We’ve never had an opportunity quite like this where everything is really intact and guarded and it’s not a salvage operation,” Brady said. “So, we want to go slowly and make this a model cave project.”

Balamkú is located about 80 feet beneath the ruins of Chichén Itzá, an important Yucatán hub that flourished from 600 to 1200 AD and is famous for the El Castillo pyramid, located about 1.7 miles from the cave.

Read More: The Ruins of a Massive Ancient City Have Been Discovered in Guatemala

Chichén Itzá sits atop a complex subterranean water system that includes sinkholes called cenotes. These bodies of water provided drinking water to ancient peoples, but also functioned as ritual spaces for ceremonies and sacrificial offerings.

The religious significance of the site is demonstrated by a “sacbe,” a white road that connects the sinkhole to the main city. Mayans reserved sacbes for the most sacred locations in their communities.

“The sacbe indicates that this was an extremely important site,” Brady said.

Interior of the cave. Image: Karla Ortega, Great Mayan Aquifer Project

The researchers have already entered and mapped out many of the cave’s claustrophobic passageways above the water table, squeezing through crawl spaces only 40 centimeters wide in some places. Eventually, they plan to explore the underwater portions of the cave as well.

“It’s never over until you come to the last dead end,” Brady said, “and we’re not there yet.”