On the morning of August 27, 1883, British colonial officers on Rodrigues Island, a speck in the Indian Ocean, heard what they logged as "the distant roar of heavy guns." They assumed a naval battle was underway somewhere over the horizon. In Perth, Australia, people heard the same booming and sent out search parties looking for a ship in distress.

There was no battle and no ship. What they heard was a volcano called Krakatoa, located between Java and Sumatra in Indonesia, tearing itself apart.

Rodrigues Island is 3,000 miles from Krakatoa. That's roughly the distance from New York to Dublin. The sound had been traveling through the air for four hours by the time it arrived, and it still landed like artillery. About 8% of people heard the explosion with their bare ears. It remains the loudest sound in recorded history.

Sound is a pressure wave, air molecules moving in a ripple that spreads out from a source. Decibels measure the magnitude of sound, and the scale is logarithmic, so the numbers can be deceptive as they climb. A jackhammer is about 100 decibels. At about 130 decibels sound is painful. A jet engine at close range is 150, enough to damage your hearing in seconds.

One hundred miles from Krakatoa, a pressure gauge at a gasworks in Batavia recorded a spike equivalent to about 172 decibels. Sailors on a ship roughly 40 miles out had their eardrums ruptured by the blast wave.

Sound in Earth's atmosphere has an upper limit. Somewhere around 194 decibels, the sound wave becomes so violent that the low-pressure troughs bottom out at a total vacuum. Past that point, it’s a shockwave traveling outward, not a sound.

Beyond about 3,000 miles, the sound was too faint for humans to hear, but it didn’t stop.

By 1883, weather stations around the world were using barometers, instruments that track atmospheric pressure. Roughly seven hours after the explosion, a pressure spike swept through stations thousands of miles away. Then, days later, it showed up again. And again. The wave crossed the entire planet, converged on the far side, passed through itself, and came back. Barometers recorded it circling the Earth at least four times over five days.

Think about what that means. Long after the sound faded below hearing, the atmosphere itself was still oscillating from a single eruption, a pressure pulse lapping the globe like a ripple in a pond. Scientists describe it as the Earth's atmosphere ringing like a bell.

The eruption itself was a catastrophe by every other measure too. It collapsed most of the island and sent tsunamis over a hundred feet high into the surrounding coastlines, killing more than 36,000 people. The sound was, in a sense, the least destructive thing Krakatoa did that morning. It just happens to be the part the entire planet witnessed.

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PS — There are claims online that the sound reached 310 dB, but that is physically impossible.