Monte Roraima

At the point where Brazil, Venezuela, and Guyana meet, stands a vast plateau. The Pemon Indians of the Gran Sabana say that it is the stump of a world-size tree that held all the fruits of the world. It is surrounded on all sides by steep rock cliffs, except for a single narrow approach. Water pours over these cliffs, forming some of the highest waterfalls in the world.
On the top of this plateau are some of the largest sinkholes ever discovered. No one knows how exactly they were formed. The holes are so deep that it is impossible to climb into them except by lowering a rope more than a quarter mile directly down into them. These sinkholes are isolated from the surrounding animal and plant life, and have unique forms of life found no where else on earth.
The first explorers to climb to the plateau did so in 1884 with the help of the Royal Geographical Society of London. Their story was the inspiration for Arthur Conan Doyle's The Lost World.