ATM Cave (Actun Tunichil Muknal) is one of Belize's best-known adventures, and you can do it from San Pedro. Take the earliest water taxi to Belize City, meet your guide and drive west to the Tapir Mountain Nature Reserve. You hike a jungle trail with three river crossings, then swim into the cave.
Inside, you wade and climb through wet passages to chambers where Maya pottery and human remains still lie where they were left, including the Crystal Maiden. Cameras and phones are not allowed inside. This is the longest day trip from the island and it's physically demanding, so you need to swim comfortably and be in good shape. Lunch is included. Book online, and tell us about any health or mobility concerns before you go.