A swimming pool, swimming bath, wading pool, or paddling pool is a container filled with water to enable swimming or other leisure activities. Pools can be sunk into the ground or built above ground (as a freestanding construction or as part of a building or other larger structure), and are also a standard feature aboard oceanliners and cruise ships. In-ground pools are most commonly constructed from materials such as concrete, natural stone, metal, plastic or fiberglass, and can be of a custom size and shape or built to a standardised size, the largest of which is the Olympic-size swimming pool. Many health clubs, fitness centers and private clubs, such as the YMCA, have pools used mostly for exercise or recreation. Many towns and cities provide public pools. Many hotels have pools available for their guests to use at their leisure. Educational facilities such as schools and universities occasionally have pools for physical education classes, recreational activities, leisure or competitive athletics such as swimming teams. Hot tubs and spas are pools filled with hot water, used for relaxation or hydrotherapy, and are common in homes, hotels, clubs and massage parlors. Special swimming pools are also used for diving and other water sports, as well as for the training of lifeguards and astronauts.