Hundreds of people are dead after the Sierra Leone landslide. Here's how it happened.

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In the early morning hours Monday, a deadly landslide came crashing down and a torrent of dirty brown flood water filled the streets in the capital of Sierra Leone. According to the Red Cross, more than 600 people are missing and at least 300 are dead in the aftermath. Bodies are washing up on the beach. The president of Sierra Leone said the country is in a “state of grief.”

“The magnitude of the destruction as a result of the disaster is such that the number of victims in the community who may not come out alive may likely exceed the number of dead bodies already recovered,” said one resident of the affected area.

Though the landslide occurred in mere seconds, it was growing more likely over the past couple of weeks. Weather reports show the region has received approximately 20 inches more than what usually falls in the last half of July and early half of August.

The soil can only absorb so much moisture until it reaches a breaking point. That usually comes in the form of runoff, but in some cases, it can flow into cracks and crevasses and lubricate the ground on top of a hill. Eventually, a slab of unstable earth could lose its footing, at which point gravity takes over and the dirt and everything on top of it - including entire trees and boulders - comes crashing down.

The resulting landslide will race downward until it reaches the bottom of the hill where the ground levels off and gravity can no longer play a role, or when friction creates an opposing force large enough to counteract gravity’s downward pull.

Landslides also occur in every U.S. state, according to the USGS. The most recent major landslide in the U.S. happened in Oso, Wash., in 2014, in which 43 people perished. Similar to the Sierra Leone landslide, the Oso disaster was likely caused by the saturation of unstable soil. Since 2014, more than 10,000 tons of debris and soil have been removed from the landslide area.

Landslides tend to occur over and over in the same location or area, simply because they are geologically prone. In the U.S., the Appalachian Mountains, the Rocky Mountains and the Pacific Coastal ranges tend to be at the highest risk for large landslides.

“Any area composed of very weak or fractured materials resting on a steep slope can and will likely experience landslides,” the USGS writes. “Although the physical cause of many landslides cannot be removed, geologic investigations, good engineering practices, and effective enforcement of land-use management regulations can reduce landslide hazards.”

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