Ankarana Reserve is a small, partially vegetated plateau in northern Madagascar composed of 150-million-year-old middle Jurassic limestone. With an average annual rainfall of about 2,000 millimetres (79 in), the underlying rocks are susceptible to erosion, thereby producing caves and underground rivers a karst topography. The rugged relief and the dense vegetation have helped protect the region from human intrusion. The plateau slopes gently to the east, but on the west it ends abrubtly in the "Wall of Ankarana", a sheer cliff that extends 25 kilometres (16 mi) north to south, and rises as high as 280 metres (920 ft). To the south, the limestone mass breaks up into separate spires known as tower karst. In the center of the plateau, seismic activity and eons of rainfall have dissolved the limestone away in deep gorges, and sometimes redeposited it in ribbons of flowstone. In places where the calcific upper layers have been completely eroded, the harder base rock has been etched into channels and ridges known as tsingy.
Beginning in the 1960s, expatriate Frenchman Jean Duflos (who married locally changed his name to Jean Radofilao) did a huge amount of exploration of the cave systems and subterranean rivers of the Massif, much of it on his own or with visiting speleologists. A total of about 100 kilometres (62 mi) of cave passages within the massif have been mapped. One of the most accessible caves, La Grotte d'Andrafiabe, alone comprises at least 8.035 kilometres (4.993 mi) of horizonatal passages.