It’s a journey even Indiana Jones would have to brace himself for: an hour-long rainforest hike, a swim across an ice-cold stream into the narrow mouth of a cave, and then a slippery descent into the dark, cavernous depths where bats, spiders, and ancient scorpion-like amblypygi lurk in the nooks. But, more than a thousand years ago, these creepy crawlies were practically friendly company compared to the cave’s human occupants, who used the remote location to carry out grisly, murderous rituals.

Today, brave adventurers willing to travel more than a mile underground in less than ideal conditions to reach Belize’s Actun Tunichil Muknal cave (ATM) will find many ancient treasures at the end of their journey.

In the past two decades, archaeologists have unearthed more than 1,400 fragments dating from between 250 and 909 A.D.—the period when the “Classic Maya” kingdoms ruled a swath of Mesoamerica. The Maya ventured deeper into the three-mile-long cave as generations progressed, a path that can be tracked by the remnants they left behind.