I don't know the complete or accurate definition of a cenote but I believe they are underground caverns. The one we went to (and our guide was going to take us to another one that was even better but we didn't make it out there) was a large multi-room cavern filled 90% with fresh water and aquatic life. We went snorkeling in the cenote and saw some fish and fresh-water shrimp. This was probably the highlight of the trip for me. I loved it! The water was a little on the chilly side but nothing bad once you started swimming.