We booked the ATM Cave tour with Carlos the Caveman based on Tripadvisor reviews, and are so glad we did! Carlos was very accommodating (we were flying to/from San Pedro so were at the mercy of Tropic Airlines and weather delays), educational and passionate! Here are the highlights of our tour:
1) We were traveling with another couple and were pleased to find out that it was just the four of us on our tour with Carlos. We saw much larger groups in the cave - I swear I counted one with 12-15! The size of our group allowed us to have a very personal feel.
2) From the start of our tour it was an adventure! The landscape is so beautiful and Carlos was educating us the entire way. Less than 5 minutes into our hike we were swimming across a river. Along the hike we got to taste different plants, eat a termite or two (good protein according to Carlos) and learn about the history of the Maya. Carlos is so incredibly knowledgeable and passionate about the culture.
3) The cave itself was an incredible and mystifying experience - a workout for both your body and mind. Swimming/wading through the underground river, climbing up, down and through crevices (some a pretty tight fit!), seeing the beautiful stalactite/stalagmite formations and finally ending in the chamber that contained the Mayan artifacts and human remains. All along the way Carlos continued to educate, inspire awe of the environment (encouraging us to "Follow your heart") and guide us safely through the terrain.
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For this tour I recommend wearing a pair of hiking/tennis shoes that you don't mind getting soaked and muddy, a long-sleeve swim shirt/rash guard, swim trunks for guys/longer shorts or leggings for gals. You will be soaked the entire time. You also need to wear socks as you are required to remove your shoes to enter the chamber with the artifacts/remains. Carlos provides life jackets, helmets and headlamps.
I highly recommend booking your tour(s) with Carlos. This was one of the best excursions I have ever been on!