Crumbling Vault

The Crumbling Vault is a random spawning event in that can be found somewhere in the Stinging Winds in Act II of Diablo III.

Lore
"Once in a thousand years, when the stars align, these ruins open, but only for a short while. There is treasure rumored to lie deep below. Find it fast, or die when the dungeon seals itself for another age."

In-Game
When found, you can tell that it's the Crumbling Vault (and not The Ruins or Chamber of the Lost Idol) when there is a Treasure Hunter standing out front. Talk to him to find out more about this event. Upon entering the The Crumbling Vault, the game will warn you that the dungeon is timed. You must find the Vault Treasure Room portal before the timer expires. If the timer expires, you will fail the event and receive no reward. Reaching the vault in time will award several chests, a possible shrine, and at least one Resplendent Chest. Talking to the Treasure Hunter out front after the event will have different answers depending upon the result.

Completing the event on time will award you the "Crumbling Vault" and "Got Out" challenges, while completing it under 90 seconds awards you the "A Second to Spare" challenge. To receive the rewards, it's best to avoid fighting and simply run through the map. Upon reaching the Vault Treasure Room you may exit via the provided portal, and the entrance to the dungeon will change to reflect this.

Trivia

 * Despite the Treasure Hunter implying that you would get trapped in the dungeon when it collapses, you're able to use Town Portal or the "End Dungeon" pylon to leave. He doesn't seem to be surprised that you were able to get out.