Greetings!
Well, it just so happens that in my campaign-world of Thandor, I do have such an area. Thandor is generally the size of Jupiter, and is an enormous world, with vast seas and many huge continents.
One such enormous continent, Rhanndar, is a primordial continent ringed on the northern coast by vast mountains that rise to over 30,000 feet. many of the peaks are glaciarized year-round. Meanwhile, further down the forbidding slopes, there are a series of mountain ranges that branch off from this vast chain, further dividing regions from each other. An enormous, 1200-mile long escarpment that is 580' feet high, elevates and separates for all practical purposes one entire region of the continent from the rest of the land.
On the continent of Rhanndar, there are primordial rain-forests, where there are a host of Dinosaurs, and strange creatures. Along the great plains area, up the escarpment and into the highlands, there are vast herds of Mammoths, Woolly Rhinosceros', and other huge herd animals. There also lurk great sabre-toothed tigers, and many wondrous beasts.
In the lowlands, where the warmer jungles are, there are great herds of herbivorous dinosaurs, flying reptiles, and the rivers, lakes, and coastal areas are resplendant with strange marine reptiles, and unusual fish. On land, great Tyrannosauraus Rex's hunt all lesser creatures, and in some areas, there are Allosaurs that prowl the cooler regions of the forest. Flying reptiles nest in great canyons, and have vast nests along cliffside canyons and sheer mountainsides.
These regions are mineral rich, and also incredibly magical. There are strange magical, "curtains"--which distort direction, weather, and even time while travelling into the area. There have only been a few explorers who have ever located this continent, and most of them have died. Rumours persist of what some where to beleived to have found, and the conjectures go on, for no one has the whole picture. Clear, certain knowledge of how to even find this strange continent is rumour-plagued, and guess-work at best.
There are rumours of all kinds of strange races and creatures that supposedly live on this wild continent. The monsters are, of course, like living nightmares given form. Still, it remains a place of wonder and mystery.
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I really like prehistoric environments! They allow for one to start from scratch, and players can really explore different ideas. I shouldn't forget the DM either, because such an environment is an unusual opportunity for the DM to integrate unusual plot elements into the campaign, with different themes, and entirely different assumptions. The DM also has the opportunity to explore with different concpets for societies and cultures, and the roughness and remoteness of the environment allows the DM to easily integrate unusual rules features, strange mechanics, unusual spells, or bizarre magic items, all within a closed environment. This means that the DM can freely experiment, and if there is something drastically or even significantly wrong, then the problem can more easily be entirely fixed, replaced, or gotten rid of en masse, as desired, without extensive, problematic effects upon the larger campaign.
Semper Fidelis,
SHARK