Y-40 is the deepest indoor swimming pool in the world, according to Guinness World Record.
The pool is located in Montegrotto Terme, Padua, Italy. Its maximum depth is 41.7 metres (137 feet), and it contains 4.3 million litres (945,867 gal; 1,135,939 US gallons) of non-chlorinated, highly filtered spring water maintained at around 32 °C (90°F). It features four underwater caves and an underwater viewing tunnel for onlookers. Watch the video below.