There could be treasures down there
I've heard of a few of those villians, but only from other kids. My foster parents didn't allow any of us to watch cartoons or read comic books.
But now you are an adult. There is no excuse for maintaining such ignorance.
My pool has led lights. Last year when my husband had pool re-tiled he also had them wire in rgb leds on the sides and bottom. It's really nice swimming with the overheads off and the leds synced to the music system.
Or it could be another underground swimmig pool. Why'd you ever decide to build the fortress in a swamp, anyway?