(For related story see INDONESIA VOLCANO/ or [ID:nJAK291822])

Oct 17 (Reuters) - Thousands of evacuated residents camped out in shelters near Indonesia's Mount Kelud on Wednesday, after the volcano's alert status was raised to maximum, meaning it could erupt within 24 hours.

Here is some background on Mount Kelud and volcanoes in Indonesia.

MOUNT KELUD AND ITS DEADLY CRATER LAKE:

-- One of Java's most dangerous volcanoes, the relatively small and inconspicuous Kelud lies 90 km (55 miles) southwest of Indonesia's second-largest city, Surabaya.

-- Its summit crater lake, 1,731 metres (5,712 feet) above sea level, has been the source of some of Indonesia's most deadly eruptions -- with an estimated 15,000 people killed in dozens of eruptions since 1500. In violent eruptions, water shoots out from the lake and triggers pyroclastic flows and lahar, smothering everything in its path with boiling mud.

-- Mount Kelud's last major eruption, in 1919, killed some 5,000 people. In 1926 workers began digging drainage tunnels to lower the crater lake level. After more than 200 people were killed in a 1966 eruption a new deeper tunnel was constructed.

VOLCANOES IN INDONESIA:

-- Local vulcanologists estimate there are 129 active volcanoes scattered across the country's more than 17,000 islands. Around three quarters of the volcanoes lie on the Sunda Arc. Sweeping 3,000 km from northwest Sumatra to the Banda Sea, the arc traces the junction of tectonic plates.

-- The U.S. Geological Survey (USGS) says Indonesia has the world's largest number of historically active volcanoes -- 76. It is second only to Japan for total dated eruptions, with 1,171 compared to Japan's 1,274.

JAVA: HOTSPOT FOR INDONESIA'S MOST ACTIVE:

-- Eighty of the 129 considered active are "Type A" -- most active since 1600 -- Indonesia's Directorate of Volcanology says.

-- A quarter of all Type A volcanoes -- 21 -- are on Java island. There are 20 Type A volcanoes on Nusatenggara, 12 on Sumatra, nine in the Banda Sea, six in North Sulawesi, five each in Sangihe and Halmahera and two on Bali Island.

WORST ERUPTIONS:

1. April 10-15, 1815: Mt Tambora on Sumbawa island erupts, killing 92,000. The world's worst volcanic eruption in terms of death toll, it reduces the 13,000 feet tall mountain to 9,000 feet. The year 1816 becomes known as the "year without a summer" as volcanic ash in the atmosphere lowers temperatures around the globe, with deadly summer frosts in the U.S. and Canada.

2. Aug 27, 1883: Volcanic island Krakatoa, which lies in the Sunda Strait between Java and Sumatra explodes. Resulting tsunamis reach heights of 40 metres and kill 36,000 people on Java and Sumatra islands. Two-thirds of the island is destroyed.

MAGNETS FOR TOURISTS AND DEVOTEES:

-- Seen as sacred in some local traditions, devotees living close to dangerous volcanoes are sometimes reluctant to evacuate when warnings are issued. The crater lake of Mt Kelud, for example, is thought to be a source of peace and prosperity by some living by its slopes.

-- Thousands of tourists are also drawn to the symmetrical cones of volcanoes such as Bromo, one of Java's most active, to see the sun rise through clouds of mist and volcanic steam; and Agung, Bali's highest and most sacred volcano, which towers over the east of the island.

Sources: Reuters, US Geological Survey, Kediri Tourism (www.eastjava.com/tourism/kediri/kelud.html), Indonesian Directorate of Volcanology and Geological Hazard Mitigation (www.vsi.esdm.go.id/general_info/index.htm)

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