The islands of the Comoros Archipelago were formed by volcanic activity. Mount Karthala, an active shield volcano located on Ngazidja, is the country's highest point, at . It contains the Comoros' largest patch of disappearing rainforest. Karthala is currently one of the most active volcanoes in the world, with a minor eruption in May 2006, and prior eruptions as recently as April 2005 and 1991. In the 2005 eruption, which lasted from 17 to 19 April, 40,000 citizens were evacuated, and the crater lake in the volcano's caldera was destroyed.