Mount Unzen in extreme southwestern Japan (Kyushu) is a series of active volcanic domes which created deadly eruptions in 1792 and in 1991.

The 1792 eruption included lava flows from Mount Fugen and the catastrophic collapse of Mount Mayuyama on May 21, 1792, creating a tsunami in the Ariake Sea. The landslide and tsunami killed as many as 15,000 people in Shimabara City and the nearby areas. It was Japan's most deadly volcanic eruption.

In 1990, after a two-hundred-year slumber, Mount Unzen awoke. It produced a new lava dome at its summit and by May 20, 1991 began ejecting lava flows. On June 3, 1991, a sudden eruption and explosion of hot gases swept nearly 3 miles (5 km) from the mountain, killing 43 volcano observers and reporters.

Among the journalists and volcanologists killed in 1991 was Harry Glicken (1958-1991), who had been scheduled to observe Mount St. Helens on May 18, 1980. His replacement, David Johnston, was killed in that eruption.

an eruption in 1792, the volcano remained dormant till November 1989, when an earthquake swarm began about 20km down and 10km west of Fugen-dake. Over the next year, earthquakes continued, with their foci gradually moving towards the summit. Tremor was first noted 4 months before the eruptions. The first eruptions began in November 1990, and after an inflation of the summit area, fresh lava emerged on May 20th 1991. On June 3rd 1991, the volcano erupted violently, possibly as a result of depressurisation of the magma column after a landslide in the crater.

For a modern eruption, damage and casualty figures are high. 44 dead, 11 injured. 2,593 buildings damaged or destroyed (1,270 of them residential). Total dollar value of damage (well, yen value) was 204.7 billion yen (I have no idea how to convert yen into another currency). Transportation systems were hard hit by heavy pyroclastic flows, which made reconstruction difficult. 12,000 people were initially evacuated, but by 1993, this had been reduced to "only" 3,000 people.

In the aftermath, the nearby cities have put into effect their best preventive measures to protect city, industry, and residences from damage. It's a nice concept, and if you believe the Tommy Lee Jones - Anne Heche movie "Volcano" it looks good. In reality, the only thing that you can do to prevent damage is to build out of the way of a reasonable blast zone and build on topography where you

Activity from 150,000 years ago to the present has occurred at a number of sites around the volcanic complex, building four main domes at different times: the Nodake (70 - 150,000 years old), Myōkendake (25 - 40,000 years old), Fugendake (younger than 25,000 years old) and Mayuyama (4,000 years old) volcanic peaks. Fugendake has been the site of most eruptions during the past 20,000 years and lies about 6 kilometres (3.7 mi) from the city center of Shimabara, Nagasaki (prefecture).

Unzen's deadliest eruption occurred in 1792, with a large dacitic lava flow coming from Fugendake. The east flank of the Mayuyama dome collapsed unexpectedly following a post-eruption earthquake, creating an avalanche and tsunami that killed an estimated 15,000 people. This remains Japan's worst-ever volcanic-related disaster.

Between 1991 and 1994 the volcano generated at least 10,000 small pyroclastic flows on a near-continuous basis, destroying about 2,000 houses. From 1993 onward, the rate of lava effusion gradually decreased, and eruptions came to an end in 1995. Eruptions, or at least seismic hints of impending eruptions, began again in 2002, and actual eruptions (though small) resumed in 2008 (as stated above).