I don't know who has the pockets and the space to install Hydrofloors—mechanically operated tiles that sink into the ground slowly, revealing a swimming pool already full of water—but I want to meet them. Watch them in action:

According to the manufacturer, the system is "nearly invisible" and you can adjust the depth of the swimming pool to any depth just by using a control panel. I wonder if my downstairs neighbor will mind it if I install one. [Hydrofloors via Notcot]