Sifting through a wall of earth in aftermath of Washington mudslide

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It's a typical Saturday morning in Oso, Wash., a town of 180 nestled in the foothills of the Cascade Range. A mother is waking her children from a slumber party; a young couple is planning a summer wedding; a father is installing a satellite dish on a roof. Along State Road 530, motorists are passing the exit for Steelhead Drive.At 10:37 a.m., a piercing screech shatters the tranquil morning. Across the Stillaguamish River, a hillside as high as the Space Needle begins to drop. Going 50 kilometres an hour, three million dump trucks worth of rock, soil and trees rush toward Oso. Within seconds, the tidal wave smashes into the community.

For two minutes, mud continues to bury the town. Clay balls the size of ambulances fall from the mountainside, smashing everything in their way. Then the ground stops moving, though homes and trees continue to fall. Two minutes later, the mud begins to flow again – for the next hour, slides rumble across the valley.

"There's a mudslide and everything's gone, the houses are gone," a woman yells in a frantic call to 911. "We watched hundreds of trees come falling and there are people yelling for help. There are so many people yelling for help."

Speaking over their radios, deputies face a wall of mud as tall as a four-storey building. Stuck on either side of the 2.5-square-kilometre slide, they struggle to understand what they are facing. They hear the calls for help.

Search-and-rescue helicopters arrive within 45 minutes. Pilots see that the river is dry, blocked by mud. Crews examine the pile of debris from above, wary of the 200 metres of exposed mountainside threatening to give way. Within three hours, eight survivors will be pulled from the mud. Nearly a week later, as many as 100 bodies remain entombed. Steelhead Drive is gone.

The disaster on March 22 began, in a sense, 15,000 years ago. As the glaciers that covered North America started to retreat north, they left behind a legacy of steep hills made of loosely packed silt that is unique to the U.S.'s Pacific Northwest.

Known to locals as Slide Hill, the mountain overlooking Oso has suffered landslides every decade for much of the past century. At its base, that hill was made of an impermeable layer of clay; above it was lose soil. The local geography is always changing, as the river has eaten into the base of the mountain.

Over the past month, near-record rain has fallen on the region. Instead of the monthly average of 9.3 centimetres of rain, more than 12 cm fell in the first 20 days of March. The water was trapped above the clay, weakening the internal forces that kept the mountain together.

The reason remains unknown, but at 10:37 a.m. the soil began to slide. The top of the hill began to bulge out, the tree line on the horizon falling, as the bottom pushed into the river and towards the town, creating a mud tsunami. "Imagine the rotation of an ice cream scoop," said David Montgomery, a geologist at the University of Washington, describing what happened to the face of Slide Hill.

The same mountain gave way in 2006 in a much smaller slide. According to Mr. Montgomery, a study of the Stillaguamish valley shows landslides even larger than the one that destroyed Oso. "The hill was a perfect recipe for disaster," he said, calling the slide "inevitable."

A report from the U.S. Army Corps of Engineers in 1999 warned that a catastrophic failure on the hill was possible. But that report was prepared as part of a fish restoration project and the conclusion was incidental – it wasn't warning of loss of life, but that silting from a mudslide disturbs fish. The report wasn't forwarded to local communities.

That mud now covers the 49 structures in the centre of Oso. Standing on a pile of debris five days after the disaster, fire battalion chief Steve Mason surveyed the damage, comparing it to the scene after the eruption of Mount St. Helens. "It all kind of rolled down this corridor here," he said, sweeping his hand across the debris, towards the towering hill. "There's a lot of grieving going on."

First responders can barely tell where homes once stood. Everyone says pictures didn't prepare them for the emotional toll of seeing the shattered moonscape. "It looks like houses, trees and mud run in a blender and then poured on the ground. It's very difficult," said Master Sergeant Chris Martin, a search leader with the Washington Air National Guard.

The more than 250 members of search crews walk single file, slowly working through the debris while using buckets and hands to move the muck. Possible sites with victims are marked with GPS co-ordinates. Searchers are looking for voids under trees or building material where survivors could be waiting for rescue. After a week, the chances are slim.

Crews with search dogs walk the mud field. The work is slow-going as they sink to their waists. The dogs tire from fighting the constant suction of mud. Some searchers are armed with electronics looking for cellphone signals. Pieces of plywood have been thrown down to create a system of walkways.

Backhoes, scoops and small loaders remove debris a few centimetres at a time. Crews call it "forensic digging." Nearly a dozen searchers watch as the shovels move earth, looking for any clue of remains. The grey muck can have the consistency of fresh concrete.

When human remains are found, or the families of victims are brought to the slide, an eerie quiet sets in. All work stops. Within minutes, the beeping and clanking sounds return, joined by a chorus of chainsaws.