I ran a few sessions of Against The Unknown, a stripped down version of Gumshoe. They were all straight mystery games. No monsters or supernatural events, just people solving strange crimes.

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…