The crater lake in Kelut Volcano in Eastern Java, Indonesia, has been a killer. Even small eruptions violently throw out the lake water, creating massive mudflows down the flanks of the volcano. After the 1919 eruption which killed over 5000 people, tunnels were drilled through the crater rim to partly drain the lake. This effort has reduced the danger from Kelut, but not completely. Eruptions in 1951, 1966, 1990, taken together, killed another 250 people.
Photo copyright R.W. Decker, 1960.