Between 19 May and 21 October 1883, the volcanic island of Krakatoa (located in the Sunda Strait, then part of the Dutch East Indies—modern-day Indonesia) began erupting, lasting more than five months. On 27 August, the island had its most significant eruption, which destroyed over 70% of the island and its surrounding archipelago, the island collapsing into a caldera. The eruption of 27 August had an estimated volcanic explosivity index of 6, and is one of the deadliest and most destructive volcanic events in recorded history; the third explosion of that day, that occurred at 10:02 a.m., remains the loudest-known sound in history.
The explosion was heard 3,110 kilometres (1,930 mi) away in Perth, Western Australia, and Rodrigues near Mauritius, 4,800 kilometres (3,000 mi) away. The acoustic pressure wave circled the globe more than three times. At least 36,417 deaths are attributed to the eruption and the tsunamis it created. Significant additional effects were felt worldwide in the days and weeks after the volcano's eruption. Additional seismic activity was reported until February 1884, but any reports after October 1883 were dismissed by Rogier Verbeek's subsequent investigation into the eruption.
In the years before the 1883 eruption, seismic activity around the Krakatoa volcano was intense, with earthquakes felt as far away as the Australian Northern Territory, one of which, in 1880, damaged a lighthouse. Strombolian activity began on 20 May 1883, and steam venting began to occur regularly from Perboewatan, the northernmost of the island's three cones. Eruptions of ash reached an estimated altitude of 6 kilometres (20,000 ft), and explosions could be heard in Batavia (Jakarta) 160 km (100 mi) away, rattling windows and doors. Dust fell 480 km (300 mi) away. An excursion party sent to the island on 26 May reported that the islands were covered with a fine white dust, with explosions coming from Perboewatan every five to ten minutes. Although by this time the eruptions were calming down, some of the explosions were still very energetic, hurling pumice opposite the direction of the wind. A large crater approximately 900 metres (3,000 ft) in diameter formed, with a cavity from which issued plumes of steam. The eruptions soon quietened after the initial violence.
Around 16 June, there were loud explosions and a thick black cloud covered the islands for five days. The eruptions started gaining strength again on 19 June. On 24 June, a prevailing east wind cleared the cloud, and two ash columns could be seen issuing from Krakatau. It was also noticed that the summit cone of Perboewatan was gone, which an extensive report of a few years later attributes it to having been blown away. The seat of the eruption is believed to have been a new vent or vents that formed on that day, between Perboewatan and Danan. The violence of the ongoing eruptions caused tides in the vicinity to be unusually high, and ships at anchor had to be moored with chains. Earthquakes were felt at Anyer, Banten, and ships began to report large pumice masses to the west in the Indian Ocean. A third vent, and later many more, appeared. The eruptions continued throughout July, and their violence continued to increase until the explosions of 27 August.
In early August, a Dutch topographical engineer, Captain H. J. G. Ferzenaar, investigated the Krakatau islands. He noted three major ash columns (the newer from Danan), which obscured the western part of the island, and steam plumes from at least eleven other vents, mostly between Danan and Rakata. When he landed, he noted an ash layer about 50 cm (1.6 ft) thick and the destruction of all vegetation, leaving only tree stumps. He advised against any further landings.
By 25 August, the Krakatau eruptions intensified. At about 1:00 p.m. on 26 August, the eruption entered its climactic phase. By 2:00 p.m., a black ash cloud could be seen 27 km (89,000 ft) high. At this point, the eruption was almost continuous, and explosions could be heard every ten minutes. By 5:00 p.m., the explosions were audible all over Java. Waves approximately a metre high, which may have been produced by steam explosions, began battering the coasts of Java and Sumatra at 5:30 p.m., and continued throughout the night. Ships within 20 km (12 mi) of the volcano reported heavy ash fall, with pieces of hot pumice up to 10 cm (4 in) in diameter landing on their decks. Between 7:00 and 8:00 p.m., a small tsunami hit the shores of Java and Sumatra, 40 km (25 mi) away.
On 27 August, a series of eleven enormous explosions occurred, which marked the climax of the eruption. At 5:30 a.m., the first explosion was at Perboewatan, triggering a tsunami heading to Telok Betong, now known as Bandar Lampung. At 6:44 a.m., Krakatau exploded again at Danan, with the resulting tsunami propagating eastward and westward. The third and largest explosion, at 10:02 a.m. (some sources say 9:58 a.m.), was so powerful that it was heard 3,110 km (1,930 mi) away in Perth, Western Australia, and the Indian Ocean island of Rodrigues near Mauritius, 4,800 km (3,000 mi) away, where the blast was thought to have been cannon fire from a nearby ship. The third explosion was the loudest sound that has ever been measured. The loudness of the blast heard 160 km (100 mi) from the volcano has been calculated to have been 180 dBSPL (180 dB re 20 μPa, or 20 kPa).
Although heard at very great distances, the sounds of the largest explosions were much less noticeable, or not heard at all in close proximity to the volcano. The reasons for this are complex and still debated by volcanologists today but a possible explanation is that the sound waves may have been projected upwards to the stratosphere and reflected away from the area, in part by ash suspended in the air. Although often directly linked in popular accounts, the explosions and the tsunamis were probably caused by distinct mechanisms. The explosions may have emanated from huge pulses of gas-laden magma, whilst the waves followed later when the ejecta slumped back down into the surrounding sea. The worst of the many tsunamis that resulted were estimated to have been over 30 m (100 ft) high in places. A large area of the Sunda Strait and places on the Sumatran coast were affected by pyroclastic flows from the volcano. Verbeek and others believe that the final major Krakatau eruption was a lateral blast, or pyroclastic surge. Material shot out of the volcano at 2,575 km/h (1,600 mph). The energy released from the explosion has been estimated to be equal to about 200 megatonnes of TNT (840 petajoules), roughly four times as powerful as the Tsar Bomba, the most powerful thermonuclear weapon ever detonated. This makes it one of the most powerful explosions in recorded history. At 10:41 a.m., a landslide tore off half of Rakata's cone, along with the remainder of the island to the north of Rakata. It is still debated whether this event caused the loudest of the explosions but, either way, the collapse of the island into the magma chamber heralded the end of the paroxysmal phase.
Some islands in the western Indian Ocean, approximately 4,800 kilometres (3,000 mi) away, still heard it at nearly the same level as a gun blast. Due to the limitations of the speed of sound, it is likely that people on these far away islands did not hear Krakatau for nearly four hours after its eruption. The pressure wave generated by the colossal third explosion radiated out from Krakatau at 1,086 km/h (675 mph). The eruption is estimated to have reached 180 dBSPL (180 dB re 20 μPa, or 20 kPa) at 100 miles from the source - possibly 310 dB at the explosion site itself - loud enough to be heard 5,000 kilometres (3,100 mi) away. It was so powerful that the sound it produced ruptured the eardrums of sailors on RMS Norham Castle of the Castle Line, which was hove to off Sumatra roughly 64 km (40 mi) away, and caused a spike of more than 8.5 kilopascals (2.5 inHg) in the pressure gauge attached to a gasometer in the Batavia (correspondent to modern day Jakarta) gasworks 160 km (100 mi) away, sending it off the scale. At Batavia, the air waves burst windows and cracked walls.