Earlier this year, a team of researchers had a close call with an enormous underwater avalanche off the coast of California. This mass of rock and sandy sediments tumbled downslope within Monterey Canyon at a rate of 8 meters (26 feet) per second (around 18 miles per hour) over a distance of 50 kilometers (31 miles).

This was the first time that instruments were in place to directly record the sediment maelstrom as it flowed forth. “Smart boulders” – hi-tech sensors that roll down with the flow – were used to track the complex movements within the avalanche.

Sadly, there’s not any particularly good footage of the collapse taking place, as the camera inside the flow was engulfed quite violently by the rapidly moving sand.

The work was carried out by an international team of researchers running the Coordinated Canyon Experiment, including Dan Parsons, a professor of process sedimentology at the University of Hull.

“These events are not rare – they happen in many submarine canyons around the world – but this is the first time we have really been able to measure an event like this in this detail,” Parsons told IFLScience.

^{The formation of an underwater turbidity current. NOAA}

These underwater avalanches are technically known as “turbidity currents.” They are driven by density differences between their sediment and the surrounding fluid.

They all have the ability to steal pre-deposited sediment from their travel surface as they move over it, thereby strengthening their flow as they go. They can be triggered by earthquakes, collapsing sea shelves, or storm surges driven by hurricanes.

One of the most famous examples occurred in 1929 off the coast of Newfoundland shortly after an earthquake at Grand Banks. Although no one directly observed the current moving along, engineers noticed that their transatlantic telephone cables began to break in sequence, further and further downslope as time passed.

As the exact times and locations for each break were precisely recorded, investigators could calculate that the current was moving at around 97 kilometers per hour (60 miles per hour). In fact, it’s this damage that turbidity currents can do to modern-day fiber optics cables that make understanding them so important, among other things.

^{An example of a previous turbidity current, as seen from a vessel trapped within it. MBARI via YouTube}

The 1929 current ultimately traveled 644 kilometers (400 miles) away from the epicenter of the tremor, which puts this year’s one to shame. Still, it’s fair to say that this turbidity current was still pretty impressive.

“Some objects weighing 1,400kg (1.52 tons) – i.e. a couple of train wheels that weight more than a Ford Focus car – were moved for several km down canyon at speeds of up to 5 meters per second (11 miles per hour),” Peter Talling, a professor of sedimentology at the University of Durham, told IFLScience.

 [H/T: BBC News]