Bottle digging, i.e. looking for and collecting old bottles, typically buried or submerged.
Wife and I live on a floating house in Oregon outside of Portland. Taking my paddle board to various beaches on the river to pick up trash has become a beloved hobby of mine. Fantastic way to get exercise, get out in nature, and clean up my community.
I kept finding old, antique bottles during outings: Coke bottles from the 40s and 50s, little medicine bottles from who knows when, old beer bottles from brands no longer in business. I eventually learned that there's a whole hobby around this called "bottle digging", even an active subreddit (r/bottledigging).
I've since bought a nice snorkel and mask and have been diving for old bottles on clear days, when river visibility allows it. Thinking about taking it to the next level and getting scuba certified.
If you like finding things, you might like bottle digging.