Well, it's not actually a forest per se...but it's really cool! Tsingy de Bemaraha National Park in Madagascar contains a gigantic limestone formation (called the tsingy) which is like nothing you have ever seen. Ninety percent of the animals that inhabit this 600-square-mile refuge are species that can be found no where else on Earth!
A large reason for this is because so much of the tsingy is difficult to access. Most of the limestone has dissolved into jagged rock formations quite dangerous for humans to traverse. This unique topography has protected its animals from human interference over the centuries, and even now previously unknown species are being discovered within the tsingy!
Check out this photo gallery from National Geographic to see the unusual rock formations and wildlife that can be found in the "stone forest". (source)