A newfound submarine landslide from 60,000 years ago is the most colossal event of its kind ever discovered.

The flow of sand and mud rushed some 930 miles (1,500 kilometers) down a slightly sloped seafloor, with an initial burst of speed estimated at 45 mph (20 meters per second), said study leader Peter Talling of the University of Bristol.

The slide occurred off the African coast.

"What causes huge submarine landslides is still being hotly debated," Talling told LiveScience. "A very large earthquake occurred in the area we studied in the 1960s without triggering a large underwater landslide. We cannot be sure how this landslide was triggered."

Why we care

Submarine landslides are of interest to scientists because they're known to sometimes generate huge tsunamis. Little is known about how the biggest of these events play out, however.

"It was one of the largest movements of material ever to occur on our planet," Talling said. "This mass was ten times that transported to the ocean every year by all of the Earth’s rivers. The flow was sometimes over 150 kilometers [93 miles] wide, spread across the open sea floor.”

Talling and colleagues dropped 1-ton dart-shaped weights to the seafloor in 200 locations, some as deep as 2.5 miles (4 kilometers), then winched sediment samples back to their ship to analyze them and learn how the debris had moved.

"The landslide disintegrated into a very fast-moving flow of sand and mud," Talling said. "We followed the trail left by this flow, which is a layer of sand and mud on the sea floor."

Why it stopped

The study gives scientists a better understanding of how deep-sea mudflows work—and just how far they can move.

Most of the material stayed suspended in the ocean for the bulk of its 930-mile journey until the seafloor gradient changed slightly, from 0.05 degrees to 0.01 degrees (a typical soccer field, for comparison, has a gradient of less than 1.00 degrees to allow drainage).

The study is detailed in the Nov. 22 issue of the journal Nature.