From If It’s Hip, It’s Here:

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.

The system is near invisible, flexible, rapidly adaptable and safe, being in compliance with relevant national safety standards. The concept is that the floor height of the pool may be elevated to set at any pre-selected water depth such as wading depth for children or a depth suited to those with special needs. This makes the pool itself diverse and multifunctional

The highest elevated position of the floor is that of the pool surround, at which point the water is out of sight beneath the floor. When fully elevated, the entire area becomes totally safe and secure for children to use. Additionally, since the pool is fully closed, energy costs for heating are greatly reduced.

Hydrofloors…or you can just keep using your white-trash pool cover…plebeian.

Thanks to the Sawed Off for the tip.