Obscure Hobbies

I've always found obscure hobbies interesting. I like the notion that
we live in a sufficiently large and rich world that there can be a
great many hobbies people participate in that can each have a
substantial following, but that the vast majority of people are
completely unaware. For some time, I've been meaning to catalog a
list of interesting "obscure hobbies", and this seems as good a place
as any to do it.

I'd be very interested in hearing from other people about items to potentially add to the list. Here's the definition I'm using for obscure hobby:

• It must have a substantial following. At least hundreds, but ideally thousands of people worldwide.
  
• It must be an "open" hobby, that anyone who decided to become involved in, could.
  
• It must be something the majority of people have not heard of and don't know exists.
  
• It must involve doing something other than collecting. While I'm sure there are lots of obscure things people collect, this doesn't quite count.
  
• It must not simply be "competetive X" where X is a well known activity that is not usually done competetively.
  

Of course, there's a lot of borderline cases and I'll probably be inconsistent in my inclusion or exclusion of particular things, but I'm ok with that.

Some examples of things that don't quite make the cut for "obscure"
(but are still interesting): Scrapbooking, Curling, Model Railroading,
Competetive Scrabble, Wheel Throwing (pottery), LARPing, Knitting and Linux Development.

Over the past few years though I've identified several that do make
the list. I'll try and write more detailed entries regarding some of
these at some point.

German Board Games

This almost doesn't count, but I am including it. People are aware of board games, but people are largely unaware of the entire genre of adult strategy games, "designer games" or whatever you want to call them. They are substantially less obscure now than they were 10 years ago, but still they're pretty obscure.

Disc Golf

If you haven't heard of this, it's a game with rules essentially like golf, but played with specialized frisbees where the "holes" are in fact metal baskets. It is played more casually than competetively, and the discs aren't quite the same as frisbees. This one is also rising out of obscurity, but it counts.

Fly Ball

A team sport for dogs which is essentially a relay race with hurdles and tennis balls.

Change ringing

You know those huge arrays of bells in churches, particularly in England and New England? The process of ringing them in subtly controlled sequences by adjusting the way people pull on the ropes attached to the bells to produce precisely scheduled peals is called change ringing.

Puzzle hunting

This one almost doesn't count since there is a fairly high degree of awareness of these. These are "hunts" not unlike a scavenger hunt which are solved by finding solutions to many puzzles with interlocking answers, often producing additional layers of puzzles. The MIT Mystery Hunt was my introduction to this kind of activity, but many others exist.

So, what should I add to the list?