Isle of the Ape
Old, old school Dungeons & Dragons neckbeards will recall that, in the original editions of D&D, Gary Gygax created a bunch of adventure modules. Isle of the Ape is in fact the very last module he wrote before he abandoned the sinking ship of TSR.
Although not as famous as the Tomb of Horrors, it makes reasonable claim to being even harder than the Tomb was -- we're talking a module intended for level 18+ characters! The Tomb gets more attention because it was designed for (and played at) D&D tournaments/conventions, whereas Isle of the Ape was basically just a module that got publicly sold.
Like a lot of 1e stuff, it was pretty shamelessly ripped off. In this case, from King Kong. Following a quest given to you by Tenser, you get sent to - and subsequently trapped in - a prehistoric monster-infested island in a pocket dimension and have to kill a giant gorilla. In 3.5, it was claimed that the Isle of the Ape and the Isle of Dread were pretty much the same thing.
Here, have a description of the Isle:
From a distance, the Isle of the Ape appears to be a pile of jagged mountains sprinkled with smoking volcanoes. At night these cones give the place a dim, hellish glow. Of course, fog and clouds enshroud the place most of the time, so only portions of the island can usually be seen, and then only from relatively close proximity.
The central mass is a gradually sloping base, a saucer, if you will, where the daily downpour of rain collects to form a large lake and surrounding swamp. This slowly drains because the water has managed to cut a bed that leads underground and empties via a 200-foot-long waterfall on the west coast of the island. The whole place is very warm, and it is muggy and steaming hot in the central morass of swamp and jungle.
Jungle is a combination of rainforest, with attendant huge trees, and true jungle. The entire place is a riot of huge mosses and great ferns, with every imaginable sort of palm and cycadeoid, vine and liana filling the spaces between the larger growths. Where water fills low spots mighty rushes and towering reeds spring up. Far overhead are many small lizards, snakes, and toothed birds–as well as pterodactyls of all sizes. Lower down are somewhat larger reptiles and all sorts of flying and crawling insects. At ground level the same is true. Everywhere there are all forms of living things – insects, invertebrates, reptiles, and the ponderous herbivorous dinosaurs hunted by the swift carnivorous ones.
Also a link to a free, online PDF version of the module: http://gobbi.free.fr/scenarii/WG6%20-%20Isle%20of%20the%20Ape.pdf