In 1858, the first transatlantic telegraph cable was laid. It worked slowly—taking 2 minutes to transmit each character of a message.

But that was a lot faster than the old way: ships, which took at least 10 days to make the journey.

That first cable failed after 3 weeks, but it showed it could be done. By 1900, there were a dozen cables connecting North America and Europe.

This ground to a halt in 1929 when all 12 cables broke in one day.

No one could figure out how it had happened. But specialized ships quickly began repair work. It took them nearly a year.

In the 1950s, scientists finally determined the culprit: a deep ocean earthquake had caused marine sediments on the slope of the continental shelf to break loose.

This created an undersea avalanche of mud and water, called a turbidity current, that raced downslope for more than 600 miles, snapping the cables as it went.

The finding encouraged operators to bury new cables beneath the seafloor to protect them.

Today more than 400 undersea fiberoptic cables traversing more than 700,000 miles carry 98 percent of international internet and trillions of dollars of financial transactions every day.

In 2020, another undersea avalanche off the Congo River delta damaged cables connecting West and South Africa. A sign that critical undersea communications are still at risk.