During the last big blast, in 1994, authorities raised the alarm one hour before it blew. At least 129 are considered active. Krakatau's 1883 explosion was heard 2,500 miles away in central Australia. Tambora blew apart in 1815. For months it choked the atmosphere with ash and dust.
Scientists say there is nothing to prevent a similar blast from happening again.
Only next time, the death toll could be much higher, because Indonesia's population has soared to around 210 million. In 1921, Indonesia's former Dutch colonial rulers established the first volcanic monitoring stations in an effort to forecast eruptions and reduce death tolls. Observation posts now operate at 60 of Indonesia's most dangerous volcanoes. Their eruption warnings have saved countless lives. On Merapi, a team of 63 volcanologists works around the clock.