Tour operators promote this tour as a must-see, which is likely why it's always packed. Every 15 minutes a group sets off on the 351m (1,170-ft.) trail into the 69m-deep (230-ft.) cave. The calcium formations are certainly cool -- amazing stalactites, stalagmites, stone flowers, and dish-shaped travertines. The lake at the bottom is an eerie clear blue; it's also fenced off to prevent people from sticking in their little fingers or toes. While you make your way back up, the next two groups will already be on their way down. From mid-December to mid-January, from 8:30 to 9:30am, the sun hits the lake directly -- an impressive sight.