You know what'd be really cool? If some kind of reaction with the concrete made the air inside the vault highly acidic and when they opened it, there's only be a fine pile of residue left.
Then, as everyone is scratching their heads, the giant ants' trap will spring. They will spew forth from the vault, ripping the heads off all the nearby spectators. People for miles will hear this creepy clicking noise and the end will have begun.
Ill be sure to get my orkin man ready in '07.