Pool Party!: Sliding Grass Reveals Hidden In-Ground Pool
These are several videos of the hidden in-ground pool that engineer Gil Klar built in his backyard. At the push of a button (hopefully a giant red one labeled 'NUKES') the top astroturf deck retracts under the other deck, revealing the pool. No word what happens if you're still in the pool when the top is re-covered, but I'm pretty sure I saw that in a James Bond movie once. So, based on the film, the answer is escape, have sex with some hot chick, nab the bad guy, then more sex with the hot chick right before the credits roll.
Keep going for three videos, the last of which shows how it was made.
Thanks to JoAnne, who had such a badass hidden pool installed in her yard she can't remember where it is. Maybe under the swing-set? Try turning the garden hose on and see if anything happens.