Just before sunset on April 5, 1815, a massive explosion shook the volcanic island of Sumbawa in the Indonesian archipelago. For two hours, a stream of lava erupted from Mount Tambora, the highest peak in the region, sending a plume of ash eighteen miles into the sky.

More than eight hundred miles away, Thomas Stamford Raffles, the lieutenant-governor of Java, heard the blast at his residence and assumed it came from cannons firing in the distance. Other British authorities in the region made the same mistake. Fearing a neighboring village was under attack, Raffles dispatched troops to find and repel the invaders.

240 miles North of Tambora on the southwestern tip of Sulawesi, the commander of the Benares, a cruiser of the British East India Company, reported "a firing of cannon." The explosions appeared to come from the South. As they continued, "the reports seemed to approach much nearer, and sounded like heavy guns occasionally, with slighter reports between." Assuming that pirates were in the area, the Benares went to sea and spent the next three days scouring the nearby islands for any signs of trouble, but found nothing.

British authorities might have been excused for assuming that the threatening sounds came from potential enemies rather than the Earth itself. They were not yet accustomed to the frequent volcanic eruptions that plagued the Indonesian islands. Britain had gained control of Java and the surrounding islands less than four years earlier, when British troops had overwhelmed that vastly outnumbered French armies who had themselves only held the area for a short time, having taken it from the Dutch when France conquered the Netherlands in 1794.

Responsibility for British policy in the area lay squarely with lieutenant-governor Raffles. The general governor of India, Lord Minto, had famously given the advice to "do as much good as we can" while governing any occupied territory. Heeding Minto's advice, Raffles took great care in understanding the peoples of the region, and their territory. He had frequently toured islands in the region and recorded his observations of geological phenomena.

So Raffles curiosity was greatly stirred when the cannon like explosions from the Southeast continued throughout the night of April 5 and into the morning hours. Shortly after dawn, a light rain of ash provided evidence that a volcano had erupted somewhere in the region.

A fog of ash drifted across the island of Java, the sun faded. The warm, humid air grew stifling, and everything seemed unnaturally still. Explosions continued for the next few days. Volcanic ash continued to fall, but in diminishing quantities. Relieved, Raffles returned to his routine administrative duties.

Just the Beginning

Around seven o'clock on the evening of April 10, 1815, Mount Tambora erupted once again, this time much more violently. Three columns of flaming lava shot into the air, meeting briefly at their peak in what one eyewitness termed "a troubled and confused manner." Almost immediately the entire mountain appeared to be consumed by liquid fire, a fountain of ash, water, and molten rock shooting in every direction. Pumice stones, some walnut sized and some as large as a fist, rained down upon the village of Sanggar, nineteen miles away. After an hour, so much ash and dust had been hurled into the atmosphere that darkness hid the fiery mountain from view.

As the ash clouds thickened, hot lava racing down the mountain slope heated the air above it to thousands of degrees. The air quickly rose, leaving behind a vacuum into which cooler air rushed from all directions. The resulting whirlwind tore up trees by their roots and swept up men, cattle, and horses. Virtually every house in Sanggar was leveled. The village of Tambora, closer to the volcano, vanished under a flood of pumice.

Cascading lava slammed into the ocean, creating tsunamis fifteen feet high. Explosions from the reaction of lava with cold seawater threw even more ash into the atmosphere, and created vast fields of pumice stones along the shoreline. These fields, some of which were three miles wide, were light enough to float. They drifted out to sea, where, like giant ice burgs, remained a hazard to ships for years after the eruption. The British ship Fairlie encountered one such field of debris in the South Indian Ocean in October 1815, more than 2,000 miles West of Tambora. The ship's crew reported the sea to be covered with them for the next two days.

The eruption destroyed the top 3,000 feet of the volcano, blasting it into the air. It left behind a crater three miles wide and a half a mile deep.

Into the Atmosphere

Propelled by the force of the eruption, grey and black particles of ash, dust, and soot rose high into the atmosphere, some as high as 25 miles above the peak of the mountain. From here the winds began to spread them in all directions.

The ash cloud took the shape of a mushroom, with the still erupting Tambora as the fiery shaft. The lightest particles in the cloud retained their momentum and remained high in the air, some even continued to rise.

By eleven o'clock that day more explosions commenced. Forty miles East of Tambora in the town of Bima, a British resident reported the blasts to sound like "a heavy mortar fired close to his ear." A rain of ash poured down on the village, crushing houses. Waves surged in from the sea, ripping fishing boats from their moorings and tossing them far onto the shore.

Explosions continued throughout the night.

On board the Benares, some 300 miles away in Makassar, the crew reported blasts so intense that they rattled the ship. The next morning, the crew began to sail South to determine the cause of the blasts. But the sky troubled the Benares's captain. "By this time," he writes, "which was about 8 A.M., it was apparent that some extraordinary occurrence had taken place. The face of the heavens to the Southward and Westward had assumed the most dismal and lowering aspect, and it was much darker than when the sun rose." Three hours later, nearly the entire sky was blotted out.

By this time, Tambora's mushroom ash cloud extended for more than 300 miles. By Noon, darkness was complete.

Finally by Noon on April 12, two days later, a faint light broke through, and the captain of the Benares was struck by the image of his ship covered with ash and pumice stone.

Twenty-four hours after Tambora erupted, the ash cloud had expanded to cover an area approximately the size of Australia. Air temperatures in the area plummeted dramatically, by as much as 20 degrees Fahrenheit by some accounts. Then a light Southeasterly breeze set in, and over the next several days most of the ash cloud drifted over the islands Northwest of Tambora. By the time the cloud had finally dissipated, villages within 20 miles of the volcano were covered with ash nearly 40 inches thick. Those a hundred miles away found 8 to 10 inches of ash on the ground.

By the time the volcano had subsided, Tambora had released an estimated 24 cubic miles of molten rock as ash and pumice. Enough to cover 10,000 square miles to a depth of 12 feet. This makes it the largest known volcanic eruption in the past 2,000 years.

The Horrific Results

Geologists measure eruptions by the Volcanic Explosivity Index. A scale of 0 to 8 is used to represent the amount of ash, dust, and sulfur a volcano throws into the atmosphere. It is a logarithmic scale, meaning that each step along the index is equal to a tenfold increase in the magnitude of the eruption. Tambora merits an index score of 7, making it approximately 1,000 times more powerful than the Icelandic volcano EyjafjallajOkull, which disrupted trans-Atlantic air travel in 2010, and was rated at 4.

Modern scientists identify and measure past eruptions using layers of volcanic debris found in ice cores, lake sediments, and other undisturbed soils.

Nearly 12,000 natives died within 24 hours of the eruption. Nearly all as a result of ash falls and pyroclastic flows. Fewer than 100 people survived within the immediate vicinity of Tambora.

On April 19, the Benares reached Bima. The coastline was barley recognizable. What had once been a beautiful harbor was now an obstacle course, littered with masses of black pumice stone, burnt and splintered trees, and the remains of sunken ships which had been hurled ashore.

When the Benares departed several days later, it sailed past Tambora. The mountain had been one of the highest peaks in the archipelago. Clouds of smoke and ash still obscured the volcano's peak. Even at a distance of six miles, sailors could see patches of lava streaming down the mountain.

A heavy rainstorm on April 17 probably saved many lives. It washed ash off of local crops and cleared the air somewhat, as well as providing fresh drinking water. Still, thousands more perished. Some from sever respiratory issues, others from diarrheal disease, a result of drinking water contaminated by the acidic ash. The deadly ash still poisoned many crops, especially in the rice fields, raising the death toll higher.

The death toll reached approximately 90,000 in Indonesia alone.

In addition to the millions of tons of ash, the force of the eruption threw 55 million tons of sulphur dioxide gas more than 20 miles into the atmosphere. There, the sulphur dioxide combined with readily available hydroxide gas. Which, in liquid form, is commonly known as hydrogen peroxide. This formed more than 100 million tons of sulfuric acid. The sulphuric acid condensed into minute droplets, each 200 times finer than the width of a human hair, that could easily remain suspended in the air as an aerosol cloud. The strong stratospheric jet stream quickly accelerated the particles to a speed of about 60 miles per hour, blowing primarily in and East-to-West direction. The aerosol cloud circumnavigated the Earth in two weeks.

The cloud spread around the Earth. While it quickly spread around the equator, it likely took more than two months before it reached the North and South poles. Eventually, the particles coalesced into a single, coherent cloud that covered the Earth.

And there it remained for some time. Had the aerosol cloud ascended only into the lowest part of the atmosphere, the troposphere, where clouds form, rain would have soon cleansed the ash from the air. But in the more stable stratosphere, conditions mostly prevent the formation of clouds of water droplets. With no clouds, there could be no rain to wash away the stratospheric aerosol veil. Only the slow action of gravity and the occasional circulation of air between the troposphere and the stratosphere could drag the droplets back to Earth.

And so the extraordinarily fine sulfur particles from the Tambora volcanic eruption that reached the stratosphere remained suspended their for years, freely transported around the globe by the winds. By the Winter of 1815-1816, the nearly invisible veil of ash covered the globe. Reflecting sunlight, cooling temperatures, and wreaking havoc on weather patterns.

The coming Winter would be one of the coldest on record. Excessive rain, frost, and snowfall would persist across much of North America and Europe in the Summer of 1816, in what would come to be known as The Year Without a Summer.