Underwater River - Cenote Angelita in Mexico's Yucatan Peninsula. The river is in a limestone cave that is flooded. The river within the flooded fresh waters of the cave is a layer of hydrogen sulphide 1 meter thick that separates the upper 30 meters or so of freshwater from the 30-60 meters deep of saltwater below.
In 1993, the Basque sculptor Eduardo Chillida, received a commission of the Canary Islands for a monument / sculpture in Tindaya Mountain, on Fuerteventura. Chillida's idea was to create a large empty void inside, with dimensions 50 x 50 x 50 meters, with an entrance tunnel of 200 meters and two skylights.