We created a "trap door" in our apron when we did "Guys and Dolls" and needed a manhole for the guys to disappear into.  It was a wonderful effect, the audience being very surprised when the lid was lifted and Sky and Niceley started down the hole.    It is just a circle cut out of the stage, and carefully reinforced with bracing below so that it doesn't slip or fall.

We haven't really had the need for one since that show, however.