I'm researching the topic of what some call 'extreme' caching - but more broadly, anything that can be considered unusual terrain. is there anyone out there with an general or specific guidance on where folks go for that sort of thing?

For example, geocaching with scuba gear, anyone? I'm also thinking about writing up some notes I have on boat caches - there's some in the Lake District, along the Medway, and on various parts of the Nene, for example. I'll mention caving but that's a fraught subject and won't send people in any specific direction!

The idea is to write up some explanations and guidance in Seeker - sort of explaining what is out there and being done, without actively encouraging newbies to jump off cliffs, get buried in caves or drown themselves in lakes...

Any thoughts?