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.