Mr. de Anda and his colleagues were exploring that system of waterways when they found the artifacts, which date to around 700 to 1000 A.D., about 80 feet underground. The team has explored about 1,500 feet so far, crawling between various chambers connected by narrow tunnels.

“The place is extraordinary,” Mr. de Anda said in a video produced by the institute, speaking as he crawled through a dark, narrow passageway.

“Now comes a stage of documentation, protection and conservation of this marvelous and unique place.”

Local residents told the authorities about the cave more than five decades ago, but it was not studied extensively at the time, the institute said in its statement. Instead, the archaeologist Víctor Segovia Pinto ordered the entrance to be sealed, ensuring that its contents remained undisturbed. Last year, Luis Un, 68, who as a child was among the residents who had told officials about the cave, led Mr. de Anda’s team to its entrance again.