Underwater avalanche continued for two days
The event would have gone unrecorded were it not for the fact that the slide broke two submarine telecommunications cables, slowing the internet and other data traffic between Nigeria and South Africa in the process.
And also because of the prescient action of researchers who had lined the length of the Congo Canyon with instruments capable of measuring current and sediment velocities.
"We had a series of oceanographic moorings that were hit by the event, which broke them from their seafloor anchors so that they popped up to send us an email," said Prof Peter Talling from Durham University, UK.
"This thing gradually got faster and faster. Because it erodes the seabed as it goes, it picks up sand and mud, which makes the flow denser and even quicker. So, it has this positive feedback where it can build and build and build," he told BBC News.
The underwater avalanche - more properly called a turbidity current - was initiated on 14 January last year. It's only being reported now because scientists needed time to recover the sensors and fully analyse their data.
