Actun Tunichil Muknal – the Cave of the Stone Sepulchre – is one of the most unforgettable and adventurous underground tours you can make in Belize. The guided trip into ATM takes you deep into the underworld that the ancient Maya knew as Xibalba. The entrance to the 3-mile-long cave lies in the northern foothills of the Maya Mountains.

Most people arrive on a guided tour from San Ignacio, Belmopan or the coastal resorts, but it's also possible to arrange a guide and self-drive.

The experience is moderately strenuous, starting with an easy 45-minute hike through the lush jungle, crossing Roaring Creek three times (your feet will be wet all day). At the wide, hourglass-shaped entrance to the cave, you'll don your helmet, complete with headlamp. To reach the cave entrance, you'll start with a bracing swim across a deep pool (about 15ft across), so you must be a reasonably good swimmer (or request a life jacket). From here, follow your guide, walking, climbing, twisting and turning your way through the blackness of the cave for about an hour.

Giant shimmering flowstone rock formations compete for your attention with thick, calcium-carbonate stalactites dripping from the ceiling. Phallic stalagmites grow up from the cave floor. Eventually you'll follow your guide up into a massive opening, where you'll see hundreds of pottery vessels and shards, along with human remains. One of the most shocking displays is the calcite-encrusted remains of the woman whom Actun Tunichil Muknal is named for. In the cave's Main Chamber, you will be required to remove your shoes; wear socks to protect the artifacts from the oils on your skin.

The trip takes about eight hours from San Ignacio, including a one-hour drive each way. A number of San Ignacio–based tour companies do the trip for around BZ$190 per person, including transportation, admission, lunch and equipment. You must be accompanied by a licensed guide (of which there are around 30). Cameras are no longer allowed inside the cave due to an incident involving a clumsy traveler, a dropped camera and the breaking of priceless artifacts.