The deepest pool in the world, known as Nemo 33, "is filled with 2,500,000 liters of non-chlorinated, highly filtered spring water maintained at 30 C (86 F) and contains several simulated underwater caves at the 10 m depth level." Video after the break. Click here for first picture in gallery.
The pool itself consists of a submerged structure with flat platforms at various depth levels. The pool has two large flat-bottomed areas at depth levels of 5 m (16 ft) and 10 m (32 ft), and a large circular pit descending to a depth of 33 m (108 ft).[via Toxel]