We've shown you some clever ways to hide your valuables, but this secret compartment, built into the back of an IKEA Lack floating shelf, takes the cake. Not only is the safe hidden, but the compartment itself can't be opened unless you have a magnet to act as a key. Here's how it works.
Instructables user Renard_Bleu walks you through his creation in the video above, including how he hid the magnetic key -- a neodymium magnet -- right on top of the shelf in plain sight. When he wants to open the safe and get to what's inside, he simply lifts the shelf off the wall, attaches the magnetic key to the safe door, and the door opens. When he's finished, he takes the magnet off the door, puts the door back into place, and puts the shelf back on the wall and the magnetic key in its container.
Even better, he rigged a makeshift alarm out of one of those speakers you'll find in birthday cards to sound every time the door is opened, so he'll know if someone who's in the know on the trick has opened the safe without him around -- the built-in speaker will start blasting "Shake Your Groove Thing" across the room. The door is secured with magnets from an old hard drive, but the great thing about it is everything you'd need for this project is easily available, and the result is pretty cool. Hit the link below for a full walkthrough with photos.
IKEA Shelf Safe with Magnetic Key [Instructables]