While on vacation in Cancún, my wife booked an excursion to Rio Secreto, which is about a 2 hour bus drive from our hotel. It was so worth it. They discovered a cave system that can be explored by tourists. Some time ago, we went to Ruby Falls in Chatanooga, TN and this blows that away. With a patient guide, we spent an hour and a half swimming, wading and walking through the incredible formations in the cave. This is highly recommended.