It's a part of the backstage tour at MGM. You board a tram and ride through the back lot, then enter an area where they are supposedly finished filming for the day. But, surprise!!!! you are suddenly in a flash flood and an earthquake...and then you ride around the back and see how it was all done. It's lots of fun and not very scary, unless you have a particularly timid little one, in which case you can explain that it's all pretend before it happens. I always liked it very much.
Oh, and also giant fireballs erupt, too.