A lever somewhere in the dungeon unlocks it and let's you open the uberchest. Though it has been stated that sometimes the level contains no such lever for unlocking the chest.
Yup, a switch somewhere on the same floor as an uberchest will unlock said chest. Sometimes there is no switch, in which case you can't open it. Pay attention to your mini-map. A locked uberchest has a different image than an open uberchest (an open one has a light blue line in the middle). So if you have locked chests, watch your map when you hit a switch and you can see if it opened. There is also a known bug that if you hit a switch to unlock a chest, save and exit the game, then reload, sometimes the game will "forget" you unlocked the chest and you won't be able to re-open it.
