I'm sorry to hear about that flood.I had The Night Below back in the day - it was one of the casualties of the basement flood. I've never been overly excited about that one for some reason. I'm also not a huge fan of mega dungeons, so that might have something to do with it.
I never really thought of it as a megadungeon (even though it's on the box). It feels more like a subterranean sandbox with several factions involved. Lots of overworld stuff going on too.
My interest in it is mostly academic, since it's considered a classic adventure - sometimes put up there with The Enemy Within. I was trying a 5e conversion of it until COVID stopped our group.