In April 2005, the Directorate of Volcanology and Geological Hazard Mitigation raised an alert, forbidding visitors from going up the volcano. Sensors on the slopes of the two mountains - Anak Krakatoa on the southern tip of Sumatra Island and Tangkuban Perahu in Java - picked up an increase in volcanic activity and a build-up of gases, said government volcanologist Syamsul Rizal. On the mountain's northern flank is Death Valley, which derives its name from a frequent accumulation of poisonous gases.