An enormous underwater landslide 60,000 years ago produced the longest flow of sand and mud yet found on Earth.
The landslide off the coast of north-west Africa dumped 225 billion metric tonnes of sediment into the ocean in a matter of hours or days.
The flow travelled 1,500km ( 932 miles) - the distance from London to Rome - before depositing its sediment.
The work, by a British team of researchers has been published in the academic journal Nature.
The massive surge put down the same amount of sediment that comes out of all the world's rivers combined over a period of 10 years.
After blocks from the original landslide disintegrated, the sand and mud travelled hundreds of kilometres suspended in the water, without depositing any sediment on the sea floor that it had passed over.
Dr Talling likened this to avalanches in which the snow travels downslope in huge clouds.
A tiny drop in the sea-floor gradient (from 0.05 degrees to 0.01 degrees) eventually forced the flow to settle into a cohesive mass.
In places, the flow was over 150km ( 93 miles) wide, spread across the open sea floor.
"If you look at the distance it travelled and how much material it moved, it was at least as big as many volcanic eruptions," co-author Peter Talling from the University of Bristol, UK, told BBC News.
The discovery came from an analysis of shallow sediment cores drilled from the sea bed during a cruise in the research ship Charles Darwin off the north-west African coast.
Dr Talling worked on the project with colleagues from the National Oceanography Centre in Southampton, UK, including Russell Wynn and Doug Masson.
The study also involved researchers from the universities of Aberdeen, UK, and Bremen in Germany.
The landslide which triggered this flow was not itself the biggest known. Several that occurred off the coast of Hawaii involved the movement of more material.
The Storegga slip off the coast of Norway was also larger.
But the scale of the flow that resulted from the African slip amazed the scientists.
Understanding the cause and evolution of these infrequent undersea flows could help exploration of the sea floor.
It could, for example, assist researchers to assess hazards posed to installations for recovering subsurface oil and gas reserves - which can be worth hundreds of millions of dollars.