Paris has 2 Opéras. The newer one, bastille, doesn't look as fancy from the outside, but has a one of a kind stage moving system that allows them to have multiple stages ready and they can be moved from center stage to underground for building or rehearsals. This allows them to play different operas on different nights.
While it may seem overkill (no other theatre in the world has this capability to that scale, this theatre can almost host a couple of 747s hidden underground), it was super cool to see this on their backstage tour. Most of the pictures were underground up to 22m (many stories) below ground level:
The tour finally ended up in the main stage that was going to be used that evening after our tour: