We also went cave swimming. Apparently (see?), there are caves in this part of Mexico, filled with fresh (freezing), mineral blue water. We swam deep into the caves, where you can barely lift your head because the stalactites hang so low, but when you plunge your face below the surface - a vast new sphere opens up. I didn't know this sphere at all - it is dark and endless - every stopping point confessed a new perforation with more and more, more. There were scuba divers who saw the other worlds. All the places we only saw openings to - the scuba divers went.
This water cave is called Grand Cenote (cenote = sinkhole or well).