GURPS The Prisoner. I’ve never run it, and I’m still not sure how I’d run it, but I think about it often.

The Plain of Jars. Thousands of giant stone jars on a plateau in Laos. Many contain bodies. No one is entirely sure who built them or why.

For land environments, I can’t think of any that have been unused, but for aquatic adventures there are a few possibilities:

• Coral Reefs - a whole maze full of creatures
• Sargasso Sea - Vast mats of impassible plants containing wrecks, hidden islands, etc.
• Deep-sea vents - the closest thing we have to an alien environment on earth…