The eruption at Japan’s second tallest volcano caught scientists unaware. This may seem surprising for a technologically advanced country accustomed to volcanic activity, especially considering that Mount Ontake is among Japan’s 47 active volcanoes which are watched especially closely. It could be because, as Scientific American reports, the explosion was likely a phreatic eruption, a shallow steam explosion comprising water and heat that is nearly impossible to predict. During this type of explosion cracks can open without warning but no magma actually erupts, it's just broken-up old rock that's been obliterated. The incredible bang from the 1883 Krakatau eruption in Indonesia, heard ‘around the world’, was from a phreatic eruption. The Guardian explains: ‘Ordinarily rainwater percolates into a volcano, is heated by the hot rocks below and then rises back to the surface to form hot springs and steaming vents. But if for some reason the water becomes trapped, or a fracture brings it into contact with magma, then the excess heat turns the water to steam.’ The steam undergoes rapid expansion which can unplug the vent and blast out massive lumps of rock. Mount Ontake has five Global Positioning System (GPS) stations and one tiltmeter, all of which measure the surface deformation that would accompany rising magma. The data from these showed no change in the run-up to or during the eruption, as Toshikazu Tanada, head of volcano research at Japan's National Research Institute for Earth Science and Disaster Prevention in Tsukuba confirmed to Nature magazine. He noted, ‘In big eruptions, magma rises, and the tilmeters and GPS would pick it up. But there was no sign of this. Even though there are relatively few instruments at Ontake.' It's unlikely, according to Tanada, that they would have missed significant magmatic activity.’ Mount Ontake’s 12 seismometers for detecting seismic waves related to volcanic activity did jump from 52 on 10 September and 85 on 11 September. After that, the rate settled at 10–20 per day. However, as seasoned volcano-watchers will tell you, this spike in seismic activity doesn’t necessarily mean anything. Natures notes, ‘Increased seismic activity at a volcano, like that observed in early September, can signal a greater chance of an eruption, but often it does not. In 2011, the same type of increase was observed at Ontake without any subsequent eruption.’ Most volcanoes demonstrate erratic seismic activity. However, Tanada suggests to Nature that there are things that could be done to help avert loss of life in the future. Firstly, new instruments — in particular, gas-measuring devices — could be installed to improve monitoring and secondly, the country could establish an early-warning system, similar to the one already in place for earthquakes, which is the world’s fastest.