Because regular pools are for poor people:
Hydrofloors® are vertically movable floors -- vertically, not horizontally. The floor actually moves upward to the surface straight from the bottom. This allows you to exploit the surface area taken up by an indoor or outdoor swimming pool. The movable floors convert the associated terrace or indoor pool room for other uses such as recreation, social events, dining and dancing.Link | via
It's one of those thing you'd always wished someone had designed, and now someone has.
