This was a favorite of Houdini, and is still considered to be a difficult and impressive trick to perform today. A performer is handcuffed or otherwise shackled, and then dropped into a glass cabinet full of water.
The danger is very real, and the magician has to be able to hold their breath for a long time. The key to escaping is all about the handcuffs, though - they can be opened using a key which is pinned to the wall on the far side of the cabinet.