Krakatoa's final blast on August 27, 1883, sent a pressure wave seven times around the globe and a sound heard 3,000 miles away on Rodrigues Island; most of the roughly 36,000 dead were killed not by the eruption but by the tsunamis it launched across the Sunda Strait.

On the morning of August 27, 1883, the British ship Norham Castle lay forty miles off the coast of the Sunda Strait, close enough that more than half her crew went deaf. What her captain did not have, writing his log in total darkness at nine that morning, was any way to know that the mountain that had just gone off no longer existed.

Krakatoa (an island volcano between Java and Sumatra, known locally as Krakatau) had been erupting on and off since May of that year. A German warship, the Elisabeth, was the first to log it, its captain spotting an ash column roughly seven miles high on the morning of May 20. Nobody in the strait treated it as an emergency. Steamers detoured around the ash clouds for a better view. Locals in the coastal towns of Anyer and Merak had grown used to the rumbling. Then, over roughly twenty-six hours starting on the 26th, the eruption escalated into four enormous explosions, the last and largest at approximately 10:02 a.m. local time on the 27th, and the island tore itself apart.

How loud was the explosion, exactly?

Loud enough to be heard as gunfire 3,000 miles from its source.

On Rodrigues Island, near Mauritius in the Indian Ocean, the police chief logged what he took for the "distant roar of heavy guns" coming from the east. He had no idea he was listening to a volcano on the far side of the planet. It remains, by most calculations, the loudest sound ever documented in human history.

Instruments settled the argument that witnesses couldn't. A barometer at the gasworks in Batavia (modern Jakarta), roughly one hundred miles from Krakatoa, recorded a pressure spike equivalent to more than 172 decibels at that distance — high enough that sound was compressing into a physical shockwave rather than ordinary noise. Barographs as far away as London and Washington traced the atmospheric pressure wave passing seven times over the following five days, radiating outward from Krakatoa, meeting at the point on the globe directly opposite it near Bogotá, and bouncing back again. It was the first eruption whose acoustic and barometric signature was captured by scientific instruments around the world rather than reconstructed from memory afterward.

What actually killed 36,000 people?

Not the explosion. The water.

Estimates of the dead run from roughly 36,000 to over 40,000, and the true number is probably unrecoverable: colonial record-keeping in the Dutch East Indies was uneven, and entire villages were erased before anyone could take a census of who had lived there. The most commonly cited figure, 36,417, comes from Rogier Verbeek, a Dutch mining engineer who spent a year and a half interviewing survivors and officials for an 1885 report that helped establish volcanology as a formal science. Whatever the precise count, the pattern in the sources is consistent: the overwhelming majority drowned, not burned.

The collapse of Krakatoa's cone sent walls of water, some estimated near 120 feet high, racing across the Sunda Strait into the coasts of Java and Sumatra. Nearly 300 towns and villages were destroyed. A Dutch gunboat, the Berouw, was carried almost two miles inland and left stranded, dead, its crew all drowned in place. Ash and pyroclastic flows killed people too, particularly on Sumatra's nearby coast, but the tsunamis did most of the work.

Why did sunsets turn red for years afterward?

Krakatoa didn't just tear a hole in an island; it punched material clear into the stratosphere. Sulfur dioxide and fine ash circled the planet within weeks, scattering sunlight in a way ordinary dust and water vapor don't: it absorbs blue and green wavelengths more efficiently, leaving red and orange to dominate, especially at sunset when light travels the longest path through the atmosphere. Northern Hemisphere temperatures fell measurably, by roughly half a degree Celsius in the following year, and didn't fully recover for several years afterward. Fire brigades in parts of the eastern United States reportedly received calls about the glow on the horizon.

The connection people remember best is Edvard Munch's The Scream. Astronomers proposed in 2003 that the lurid orange-and-red sky in Munch's 1893 painting reflects a Krakatoa-tinted twilight he witnessed in Oslo, a full decade after the eruption, since the atmospheric effects lingered for years. It's a good story, and it may well be true; it is not, however, settled. Other researchers have argued the sky in the painting more closely resembles nacreous ("mother-of-pearl") clouds, a natural phenomenon visible at high latitudes that owes nothing to volcanic ash. Nobody can prove, plainly, which one Munch was looking at, and both explanations remain in circulation.

Is Krakatoa still there?

Most of it isn't. The northern two-thirds of the original island collapsed into the caldera during the final explosions, leaving open water where solid ground had been.

But the system beneath it never went quiet. Surveys detected renewed activity on the seafloor by around 1927, and a new cone broke the surface that year inside the flooded caldera. Indonesians named it Anak Krakatau ("Child of Krakatoa"), and it has been growing, erupting, collapsing, and regrowing ever since, adding roughly several meters of height most years it's active.

That child is not merely a curiosity. In December 2018, part of Anak Krakatau's flank collapsed into the sea during an eruption, triggering a tsunami that struck the coasts of Java and Sumatra with almost no warning, since the wave wasn't generated by an earthquake and none of the region's tsunami-detection buoys were positioned to catch it. More than 400 people died. Volcanologists now monitor the cone continuously, aware that the same mechanism that built Krakatoa's 1883 catastrophe, an unstable volcanic edifice standing in open water, is still assembling itself a few hundred yards from where the original mountain used to stand.

Sources

• Wikipedia, "1883 eruption of Krakatoa"
• Discover Magazine, "The Loudest Sound Ever Heard"
• NOAA National Centers for Environmental Information, "On This Day: Historic Krakatau Eruption of 1883"
• Sky & Telescope, "Astronomical Sleuths Link Krakatoa to Edvard Munch's Painting The Scream" (2004)
• Wikipedia, "Rogier Verbeek"