What hobby seems boring to most people but is actually fascinating if you dive into it?

This was a relatively short-term thing, but I think it counts loosely as hobbyish... in my 20s, I was determined to find the best bloody mary in town. I went to every single bar, restaurant, etc. and tried each one in turn.

Which sounds boring, I know, but I had so many great conversations and met so many interesting people.

And to answer your likely question: Surprisingly, Red Lobster, which made its own mix from real tomato juice and didn't use some crappy bloody mary mix. This was in the 90s, so I can't endorse their current bloody mary.