Meaning of “numa”(沼)

It means formally swamp.
But if it’s slang, it means really attractive something. (And someone will be too crazy about it.)
Imagine the swamp, if you put in it, you can’t get out.
Same with it, if you’re into in something, you can’t stop thinking about it or stop doing something about it. You can’t get out of the something.

For example, If I’m hooked on one anime, I’ll check every video of it and start to buy goods of it and I also go to watch the movie of it. Also I might check fan fiction stories and I’ll be a member of community of it, etc…
I can’t stop it!! This is numa!!!

Example Conversation

A: Actually, I like trains.
B: Oh, are you train otaku?
A: Kind of… Trains is numa.

A: 実は電車好きなんだよね。
B: あれ、鉄オタだっけ？
A: そんな感じ。電車はねぇ沼だよ。