In the Actun Tunichil Muknal Cave (or ATM Cave for short) you get to explore both a natural wonder and an archaeological goldmine. ATM is a wet cave that the Maya believed served as an entrance to the underworld. They used it for ceremonial purposes, and left plenty of evidence behind.

Your tour begins with a hike through the rainforest of the Tapir Mountain Nature Reserve. Eventually you’ll reach the river that leads to the mouth of the cave. You’ll swim for a distance 15 feet (4.5 m) to the entrance. Once you’re inside the cave you’ll walk, wade, and climb to the main attractions, so be ready for some physical activity.

On your tour you’ll see Maya vessels and artifacts, as well as the remains of sacrifice victims. The most famous of these victims is the Crystal Maiden, a young female whose skeletal remains have fused with the sparkling material that forms the inside of the cave. The skeletons are in a part of the cave known as the Cathedral, which is named for its impressively high ceiling and ornate stalactites.

These artifacts and remains are not behind any type of barriers, so be careful and wear a headlamp instead of carrying a cumbersome flashlight.