Ankarana Special Reserve is a protected area in northern Madagascar created in 1956. It is a small, partially vegetated plateau composed of 150-million-year-old middle Jurassic limestone. With an average annual rainfall of about 2,000 millimetres (79 in), the underlying rocks have been eroded to produce caves and feed subterranean rivers—a karst topography. The rugged relief and the dense vegetation have helped protect the region from human intrusion.