OK, first,
here's a link to a helpful article on Amusing Planet; the
wiki article is also good. To summarize, a tepui is a tall, very steep-sided mountain with a flat top. On our Earth, there are a bunch of them in South America, mostly in Venezuela. Due to their isolation, there are species that are only found on a single tepui, or on two close tepuis that were once connected. From above, they sometimes look like islands in a sea of clouds.
Getting to or from the top of a tepui is often massively difficult, to the point that I wouldn't be surprised if some had never been visited by modern humans. Explorers visiting them might be attacked by giant, flightless birds with a dinosaurian appearance, or meet a lost civilization, cut off from the world for millennia, or the survivors of a plane crash (or they may
be the survivors of a plane crash). Creatures with strange abilities may dwell here (catalyst creatures, Horror monsters, et cetra), or strange artifacts may be found. Also, in the absence of the paranormal, it makes for a great 'cozy mystery' setting, though a very different one from the traditional English cozy mystery.
Thoughts?