Kelud volcanic lake is one of the most active and most dangerous stratovolcano in Indonesia. Many lives were claimed by Kelud eruptions in the past six centuries mostly due to pyroclastic flows, surges and especially lahars, as in 1919 eruption (5160 victims). Before the last eruption (1990) a system of drainage was done to maintain the volume of the lake at low level (2,000,000 m3) and to control the lahars. Hydro-acoustic monitoring is the first system of surveillance that has been performed at the crater lake and has revealed intense gas bubbling a year before seismic precursor records of the 1990 eruption.
