Rio Secreto Underground River Tour with Crystal Caves. Discover the mysteries of the Yucatan on a visit to Rio Secreto Nature Reserve. Enjoy an exciting afternoon adventure at Rio Secreto, where you’ll wade and swim through an underground river and encounter the stillness of a crystal cave. You can select hotel pickup and drop-off, or meet your guide at a central location in Cancun. Book now and Save 10%: Rio Secreto Underground River Tour with Crystal Caves.