We’re all familiar with those violent shaking events we call earthquakes. But it may surprise you to know that Earth more often moves without quaking in what’s called a slow or silent slip. It sure surprised scientists: they discovered this just 20 years ago.

Earth’s tectonic plates began to move over a billion years ago. Their constant motion has formed, pulled apart, and slowly destroyed continents for millennia.

Each tectonic plate rotates around an axis, while its edges collide with, slide over, or subduct under other plates.

While some of this movement occurs in dramatic releases of stress that produce earthquakes, nearly as much energy is built up and released in slow motion—so slow it’s hard to measure.

Scientists found evidence of one slow-slip event that went on for 32 years around Sumatra, before it finally gave way catastrophically, in the tragic earthquake of 1861.

They discovered this by looking at old coral.

In times of rising sea level, corals grow upward, seeking sunlight.

In the waters around Sumatra, they found five times the coral growth rate over those three decades, revealing that the ocean floor beneath the reef was subsiding, as the Indo-Australian Plate slowly slipped beneath the Eurasian Plate.

Understanding slow-slip events like this may one day allow scientists to better predict the earthquakes that often follow them.