On Aug. 27, 1883, four huge explosions sent flows of hot gas, rocks and ash speeding across the island and caused deadly tsunamis. The eruption, which created Krakatoa's present day caldera (crater), was one of the largest in recent history (VEI 6) and is second only to Indonesia's Tambora eruption in 1815 in the number of deaths, estimated at 36,000.

Pictured: The Krakatoa cone formed within the 1883 caldera has been the site of frequent eruptions since 1927.

More info:
U.S. Geological Survey: Indonesia Volcanoes and Volcanics