Krakatoa: The Sound Heard Four Thousand Kilometers Away

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On an island in the middle of the Indian Ocean, a watch officer took the distant rumble for a ship's guns and wrote in his log, "probably heavy gunfire." The sound had come from four thousand eight hundred kilometers away, from a mountain that no longer existed. That morning the earth made the loudest sound in its recorded history.

The Sunda Strait is a narrow waterway that separates the islands of Java and Sumatra. It is one of the busiest sea passages in the world; at the close of the nineteenth century, steamships out of Europe, sailing vessels loaded with spices, and the gunboats of the Dutch colonial navy all passed through it. In the middle of the strait, between those two great islands, lay a small island that nobody paid much attention to. Roughly nine kilometres long, five kilometres wide, covered from head to foot in tropical forest. Its name was Krakatoa.

The island was not really an island; it was a single body formed where three separate volcanic cones had merged above the surface of the sea. At its northern end rose Perbuwatan, low and broad, only one hundred and twenty-two metres above the water. In the middle stood Danan, four hundred and forty-five metres. And in the south stood the master of the island: Rakata, a steep cone of eight hundred and thirteen metres, its summit more often than not inside the clouds. To sailors, Rakata was a landmark, not a threat. Passing through the strait they would look at it, correct their course by it, and then forget it.

They had reasons to forget. Krakatoa's last great rage went back to the sixteen-eighties, and even the memory of that event had grown blurred. For two centuries the mountain had slept. Its forest had returned, birds had settled, fishermen rested along its shore. There was no permanent settlement on the island, though people came over from Sumatra from time to time to cut timber from its woods. Geology was still a young science; the conceptual framework that would say a volcano falling silent for two hundred years does not mean it has died, but may instead mean that suppressed pressure is accumulating, had not yet fully taken hold in those years.

The first sign came on the twentieth of May, eighteen eighty-three. Perbuwatan, the lowest and northernmost cone of the island, woke up with a roar. In Batavia, today's Jakarta, windows rattled; the city was more than one hundred and sixty kilometres away. Ash and steam climbed into the sky. The officers of a German warship named the Elisabeth, passing through the strait at the time, recorded that the column reached a height of about eleven kilometres. That meant a serious eruption, but it did not mean a catastrophe. The Indonesian archipelago is full of volcanoes; everyone who lived in the region was used to the occasional fall of ash, to tremors in the ground, to columns of smoke in the sky.

What happened next is a scene that, looking back today, makes the skin crawl. Krakatoa turned into a tourist attraction. Dutch merchants and officials in Batavia hired boats at the weekend and organised excursions to the island. They walked on the hot ash, climbed right up to the rim of the crater and looked down into it, collected scorched branches as souvenirs. Photographs were taken. One engineer described throwing his hat into the air inside the column of steam rising from the crater's mouth. The island looked picturesque rather than dangerous.

June came, and Danan woke as well. Now two cones were erupting at once. In July, ships passing through the strait reported floating layers of pumice on the surface of the water; in some places that layer had thickened so much that vessels struggled to force a path through it. In early August, a Dutch engineer who landed on the island counted eleven separate eruption points. The surface of the island was no longer green; it was covered in a grey blanket of ash and stone dust, and the trees had been reduced to charcoal.

Even so, nobody evacuated the region. Nobody so much as considered it, because there was no way of knowing what was coming. Volcano monitoring did not exist in that era: there was no seismograph network, no gas measurement, no instrument that could show a magma chamber swelling. The only data available was what people saw with their eyes and heard with their ears. And what could be seen was a mountain that had been grumbling continuously but within limits for three months. As long as the grumbling continued, the human mind was normalising it.

By the twenty-sixth of August, tens of thousands of people were living on the two shores of the Sunda Strait. On the Java side there was Anyer, the most important port town on the strait, the terminus of the Dutch postal and telegraph line. A little to the north lay Merak, a workers' town that lived off its stone quarries. On the Sumatra side there was Teluk Betung, the administrative and commercial centre of the Lampung region, a crowded port built at the head of a bay. Between them, only thirteen kilometres from Krakatoa, sat the island of Sebesi, home to three thousand people. All of them were living an ordinary summer's day on top of a pressure swelling at the bottom of the sea.

On Sunday the twenty-sixth of August, at around one in the afternoon, the sound changed. It was no longer a grumble; it was a series of hard, dry detonations, one after another. Towards two o'clock the black cloud that appeared above Krakatoa climbed past a height of twenty-five kilometres and spread open like an umbrella. The strait went dark. Towards evening, ships some twenty kilometres away reported pumice and hot ash raining onto their decks; so hot that the sailors could not touch it without burning their hands.

That night an English sailing ship named the Charles Bal, passing through the strait, left behind a testimony that entered the historical record. Her captain and crew stayed some fifteen kilometres from the island for hours and recorded what was happening hour by hour. By their account the sky was like ink, yet it kept lighting up: lightning flickering inside the clouds, blue lights appearing at the tips of the masts, sparks running across the surface of the sea. The sound was violent enough to make it impossible for the crew to hear one another. Under the weight of the ash they were forced to clear the deck at regular intervals.

The real event came the following morning.

On the morning of Monday the twenty-seventh of August, at half past five, the first great explosion came; its source was Perbuwatan. At six forty-four came the second, this time from Danan. Then a short silence. And at two minutes past ten in the morning, the most powerful sound recorded in human history was produced.

What happened in that moment was not a simple explosion but a structural collapse. The magma chamber beneath the island had emptied, and the mountain above it fell into that void under its own weight. A mass of rock hundreds of metres thick settled, all at once, into seawater that was in contact with magma hotter than a thousand degrees. The energy released destroyed more than seventy per cent of the island. Perbuwatan and Danan disappeared entirely. Rakata was split in two; its northern half sank into the sea, its southern half remained standing as a sheer cliff. What was left behind was an enormous caldera pit descending two hundred and fifty metres below the seafloor. Roughly twenty cubic kilometres of rock, ash and pumice were hurled into the atmosphere. The ash column reached a height of eighty kilometres, far beyond the stratosphere.

The sound wave spread out of the strait, across the ocean, in every direction.

In western Java, people felt their eardrums burst. On ships sixty kilometres from Krakatoa, sailors' eardrums genuinely ruptured. The pressure gauge at the gasworks in Batavia recorded a jump in which the needle flew clean off the paper. Even one hundred and sixty kilometres from the source, the sound pressure exceeded one hundred and seventy decibels according to modern calculations; a value far beyond standing next to a jet engine.

But the sound did not stop. It kept spreading.

In the west of Australia, in Perth, three thousand one hundred and ten kilometres away, people reported hearing heavy artillery fire in the distance during the morning hours. In the centre of the continent, at Alice Springs, the same thing was heard. And the furthest range of the sound was at the other end of the Indian Ocean, on the island of Rodrigues near Mauritius: four thousand eight hundred kilometres from Krakatoa. The island's chief of police recorded that he had heard heavy rumbling from the east that morning, like the sound of cannon, and had written a report supposing it came from a naval battle. It was only weeks later that it became clear that the sound, having crossed a quarter of the world, belonged to a volcano.

This is the known limit of the range a sound can travel through the air. Beyond four thousand kilometres the sound had turned into a pressure wave of frequencies too low for the human ear to catch. But it had not disappeared.

At ten forty-one came the fourth and last great explosion. After that, what remained was an invisible wave circling the entire atmosphere. The pressure wave Krakatoa produced went around the world three and a half times; for five days it travelled the planet. From London to Buenos Aires, from Vienna to Calcutta, the barographs at meteorological stations, the instruments that draw air pressure onto paper, recorded the same small jump exactly seven times: with every pass, the wave moved the needle. Not one of those measuring stations knew what it had recorded that day. The papers were archived. Their meaning would be deciphered later.

Those who heard the sound survived. The ones who died were those who never had the time to hear it.

The collapse of the mountain into the sea pushed the water out as a single enormous mass. The Sunda Strait is narrow and shallow; the wave could not spread and dissipate the way it would in open ocean, and as it compressed towards the shore it rose. A little after ten in the morning, a wall of water more than thirty metres high struck both sides of the strait. The mark measured at Merak was above forty metres; the water had climbed as high as the houses on the hilltop.

Anyer disappeared first. It was the tidiest, most European town on the strait; it had a stone quay, a warehouse, a hotel, a telegraph office. After the wave passed, what remained was flattened ground and mud. The lighthouse on the shore was torn out of its foundation. In those same hours the water rode over the island of Sebesi; the island was low and the wave went straight across it. Of the three thousand people living there, not a single one survived.

On the Sumatra side, Teluk Betung had the worst geography of all, sitting at the very head of its bay. The narrow bay gathered the wave like a funnel and lifted it. The town was wiped out along with its harbour, its market, its Dutch administrative building and its neighbourhoods. The most concrete evidence of the wave's height there is where a ship ended up: the Dutch gunboat Berouw, at anchor in the harbour, was lifted by the water, carried two and a half kilometres inland from the shoreline, and left about nine metres above sea level on the bank of a riverbed. The ship stood there for years, inside the forest, its hull almost intact. Of her twenty-eight crew, none survived.

In those same waters, in those same hours, one ship was saved. The Dutch steamer Loudon lay at anchor off Teluk Betung. When her captain saw a wall of water approaching on the horizon, he cut the anchor chain, opened the engines to full, and turned her bow directly into the wave. This is a manoeuvre known in seamanship but requiring nerve to execute: to be caught side-on by a wave is to capsize, while entering it at a right angle gives you a chance to climb. The steamer climbed that mountain of water, passed over its back, and came down the other side. Those watching from the deck saw the town they had left behind vanish within a few minutes.

On the two sides of the strait, a total of one hundred and sixty-five towns and villages were completely erased, and hundreds more were badly damaged. The count made afterwards by the Dutch colonial administration recorded the death toll as thirty-six thousand four hundred and seventeen. Of those, twenty-one thousand five hundred and sixty-five were in the Banten region on the Java side, and twelve thousand four hundred and sixty-six in Lampung on the Sumatra side. That figure is the official count and is in all likelihood incomplete: fishing communities along the coast, unregistered villages, small boats out at sea could not be fully included in it. The overwhelming majority of the deaths came not from the volcano but from the water. Krakatoa killed people not by burning them but by drowning them.

The months that followed became a quiet continuation of the disaster in their own way. The eruption had poured millions of cubic metres of pumice into the sea, and pumice is lighter than water; it floats. Across the Indian Ocean, floating islands of stone formed, kilometres wide. Currents carried them west. For months ships struggled to push through those layers. And along with those rafts, the remains of the disaster's victims were carried too: nearly a year later, human bones were found among the heaps of pumice washing up on the east coast of Africa. The people who died in the Sunda Strait had crossed the Indian Ocean from one end to the other.

The western coast of Java could not recover for years. Salt mud and ash had covered the fertile rice fields; the soil could not be sown for several seasons. Most of the survivors migrated inland. The map of the coastline had to be redrawn, because the seafloor had changed, new shallows had formed, and old passages had closed.

The reason this disaster holds a place of its own in history is not only its scale. Krakatoa is the first great catastrophe that almost the whole world learned about at nearly the same moment.

What made that possible was the copper cable laid along the ocean floor in those years. By eighteen seventy, Batavia had been connected to the undersea telegraph network: the cable running out of the city reached Singapore, from there across India to the Mediterranean, and from there to London. A message could travel from Java to London in something like three hours. For comparison: only eighteen years earlier, when Abraham Lincoln was assassinated, the news had taken two weeks to reach Europe. A world in which news spread at the speed of a ship had given way to a world in which it spread at the speed of a cable, and that shift was so recent that nobody had yet grasped its consequences.

On the morning of the twenty-seventh of August, the telegraph line at Anyer went dead; naturally, since the town itself had ceased to exist. But the line in Batavia was working. Agents in the insurance business, correspondents for the shipping companies, and a local newsman who passed information to the Reuters agency poured the first fragmentary reports out of the strait into the telegraph. The messages went to Singapore and were distributed from there. A few hours after the explosion, London knew that something extraordinary had happened in the Sunda Strait. The next day the European papers carried the story. Within a few days, even provincial newspapers in the American midwest were publishing detailed reports.

This was a new experience. A large part of the world's population, for the first time in their lives, followed an event unfolding on a continent that was not their own. The papers printed fresh detail every day, and readers waited for the next edition. The thing we take as ordinary today, a disaster on the other side of the world entering our daily agenda, was invented precisely in those days.

That same speed also created an opportunity for science, and the opportunity was seized with surprising rigour.

The first comprehensive work carried out on the ground was done by Rogier Verbeek, a geologist working for the Dutch colonial mining administration. In the months after the catastrophe, Verbeek covered the region hand's breadth by hand's breadth, measured ash thicknesses, mapped the traces of the waves, gathered the testimony of survivors, and published a detailed report in eighteen eighty-five. That report is a turning point for modern volcanology: for the first time, a great eruption was reconstructed from measured rather than estimated data. Verbeek correctly diagnosed why the island had vanished; he described the emptying of the magma chamber and the mechanism of collapse. And at the end of his report he placed a prediction that looked bold for its day: one day a new volcano would rise out of the caldera.

In London, meanwhile, the Royal Society set up a special committee to examine the event. What that committee did amounts to an early and impressive attempt at global data collection in the history of science. Letters went out to meteorological stations, lighthouses, ships' captains and colonial administrations all over the world: what did you record on and after the twenty-seventh of August? Would you send us your barograph papers? Would you describe the colours of your sunsets? The answers flowed in for years. The resulting report, compiled under the direction of the meteorologist Symons, was published in eighteen eighty-eight: hundreds of pages of measurements, testimony, maps and colour plates showing the hues of the sky.

What that report brought to light was an idea nobody had previously held with evidence in hand: the atmosphere is a single system. Seeing the same small jump appear seven times on barographs around the world, at regular intervals following one another, proved that a pressure wave produced at a single point could travel around the entire planet. The way the sunset reports spread over the months, first westward along the equator and then north and south, revealed the existence of a continuous and fast-moving wind system in the stratosphere. The air was not a local phenomenon divided by national borders; it was a single connected covering. Krakatoa demonstrated this in measurable form for the first time.

Even after the mountain fell silent, the sky went on talking.

The sulphur compounds the eruption had flung into the stratosphere turned into a layer of fine sulphuric acid droplets up there. Such a layer is not washed out by rain; it hangs for months, even years, and scatters sunlight in unfamiliar ways. From the autumn of eighteen eighty-three onwards, sunsets became abnormal everywhere in the world. The horizon was painted in blood-red, purple and copper tones; the sky went on glowing long after darkness had fallen.

People did not know the cause, and their reactions were sometimes comic and sometimes unnerving. In November eighteen eighty-three, in the northeast of the United States, in cities such as New Haven and Poughkeepsie, fire crews went on alert and turned out: they took the redness on the horizon for the sign that a neighbouring town was burning. In those same months the sun itself grew strange; in some regions the sun and the moon appeared blue or green, because the size of the droplets in the atmosphere filtered particular wavelengths selectively. This episode is thought to have played a part in spreading the expression "blue moon," used for something out of the ordinary. Some art historians have suggested that the burning orange sky in Edvard Munch's The Scream may also have been fed by the European skies of the post-Krakatoa years; that is an interpretation, not an established finding.

Beneath what could be seen, something else was happening that could be measured. That thin layer, reflecting part of the sunlight back into space, cooled the planet. In the year following the eruption, the average summer temperature in the northern hemisphere dropped by about four tenths of a degree. The effect did not vanish at once; it took close to five years for global average temperatures to return to normal, and over that period drops of up to one point two degrees on average were measured. The numbers may look small, but a deviation approaching one degree in a planet's average temperature is large enough to shift harvest calendars, monsoon patterns and the severity of winters. A geological event that took place over a single afternoon bent the world's climate slightly for several years.

Then the Sunda Strait went quiet. The water above the caldera was deep and still. The fishermen slowly came back. That last prediction in Verbeek's report stayed on paper for forty-four years.

On the twenty-ninth of December, nineteen twenty-seven, the sea began to foam where the old island had stood. Steam and ash burst out of the water. The caldera in the depths had begun to fill again. The first ash cone that formed was worn away by the waves, and so was the second, and so was the third. But the fourth cone, which broke the surface on the eleventh of August, nineteen thirty, managed to harden itself with lava flows and remain standing. Local people named it Anak Krakatau, the Child of Krakatoa.

The child grew. Rising several metres a year, it became one of the fastest-growing volcanoes in the world; by twenty eighteen it stood more than three hundred metres above the sea. Vegetation took hold on it, birds and insects settled. For biologists it became a laboratory without equal: one of the very few places on earth that offers the chance to watch, from zero, how life returns to a sterilised island.

And it reminded the world what its father had been.

On the evening of the twenty-second of December, twenty eighteen, after months of ongoing eruption, the southwestern flank of Anak Krakatau slid into the sea as a single piece. This was not an explosion but a landslide; roughly half the body of the island settled into the water. Because it did not produce a strong enough earthquake, no seismic alarm sounded anywhere. Waves rode over the holiday towns on both sides of the Sunda Strait, over the hotels along the shore and the coastal villages, without any warning, in the dark of night. Four hundred and thirty-seven people died.

One hundred and thirty-five years apart, at the same spot, the same mechanism had worked: the weight of a mountain suddenly handed over to water. The scale of the two cannot be compared; the event of twenty eighteen is a small one beside eighteen eighty-three. But both said the same thing. Earthquake-based tsunami warning systems do not see volcanic tsunamis, because what they are looking for is a shaking of the ground, and the sliding of a mountain is silent. After twenty eighteen, Indonesia began building new systems that monitor movement on volcanic slopes and rely directly on measurements of sea level.

Today, seen from a ship passing through the Sunda Strait, the view is a calm tropical panorama: blue water, green shores, a few small islands on the horizon. One of those islands is Rakata, its southern side cut away in a sheer cliff, the surviving half of the mountain that was split in two in eighteen eighty-three. Beside it stands Anak Krakatau, low and grey, its smoke never letting up. And beneath the water between the two of them, that great pit descending to the seafloor is still there.

Krakatoa did not disappear. It only changed shape, and began to wait.

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