Nemo 33 is the deepest indoor swimming pool in the world.The pool is located in Brussels, Belgium. Its maximum depth is 34.5 metres (113 ft). It contains 2.78787 million litres of non-chlorinated, highly filtered spring water maintained at 30 °C (86 °F) and holds several simulated underwater caves at the 10 metres (33 ft) depth level.
Blue Grotto, Italy, The Blue Grotto is a spectacular accident of nature. Found on the Italian island of Capri, this soft limestone cave is suffused by an eerie blue light. The light comes from another underwater entrance to the cave and is reflected off the white cave floor. Roman emperors supposedly used the grotto as a private bath.