You investigate the machine that's supposed to produce an avalanche. It's a chamber with a disguised trapdoor painted to look like part of the mountain backdrop behind the stage. A clockwork mechanism opens the trapdoor when a handle is pulled. Sheets of metal bang and thunder against one another to produce ominous rumbling controlled by another lever.
In theory, once loaded with rocks or ice, the contents should stay safely inside until it's time for them to cascade onto the stage. In practice, that isn't working well at all.