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Y-40 "The Deep Joy" pool first opened on 5 June 2014 and was designed by architect Emanuele Boaretto.
It is 42.15 metres (138 ft) deep which, at the time of opening, made it the deepest pool in the world.
It contains 4,300 cubic metres (1,136,000 US gal) of thermal water kept at a temperature of 32–34 °C (90–93 °F).
The pool features underwater caves and a suspended, transparent, underwater tunnel for guests to walk through.
It includes platforms at various depths, ranging from 1.3 metres (4.3 ft) to 12 metres (39 ft), before the walls of the pool narrow into a well-like funnel, which plunges straight down to 40 metres (131 ft).
Hotel Terme Millepini offers tickets to freedive and scuba dive.
Italian freediver Umberto Pelizzari first measured the depth before the pool was open.
When it opened on 5 June 2014, it was awarded the "Deepest Swimming Pool for Diving" by the Guinness World Records.
That record was previously held by the Nemo 33 pool in Belgium.
Y-40 now stands as the third deepest swimming pool in the world, exceeded only by Deepspot in Poland, and Deep Dive Dubai in the United Arab Emirates.
