It took almost 1400 tons of explosives to shift the peak of Ripple Mountain.

On this day in 1958, Canadians gathered around their television sets to watch as an underwater mountain on the country’s west coast was blown up using unprecedented force. The Ripple Rock explosion would be one of the largest non-nuclear peacetime explosions ever, and it was one of the first events played live on television around the country by the Canadian Broadcasting Corporation.

The underwater mountain lay between two islands off the coast of British Columbia, north of Vancouver, in a channel used for shipping, called the Seymour Narrows. Although the water was deep, according to the CBC, at low tide the mountain’s south peak came within about 10 feet of the surface. This obstacle, invisible from the surface, posed a hazard to the bottom of ships as well as creating waves and whirlpools, the CBC writes. In total, Ripple Rock damaged or sunk 20 large ships and more than 100 small ones. At least 114 lives were lost in the wrecks.

Although the problem had been known since the 1700s, when Captain George Vancouver called Seymour Narrows “one of the vilest stretches of water in the world,” its solution was less clear. Although some groups wanted to remove the rock, writes the CBC, others thought the underwater peak could be used to support a bridge to British Columbia’s mainland.

The Seymour Narrows is part of a maze-like series of small channels and cuts that stretching between Seattle and Alaska known as the Inside Passage. It's often frequented by cruise ships today, but the spectacular coastal views and protection from open ocean come at a price, writes Michael Byrne for Motherboard. The areas are made dangerous by tidal currents. "These oceanic capillaries are where the sea breathes in and out in the form of tides," he writes. "Rapids like Seymour's are the result of a differential between tidal elevations at either end. These differentials essentially create bi-directional ocean-rivers." Add a large, jagged underwater rock to the already difficult-to-steer passage and you have a recipe for shipwrecks.

Nobody could do anything about the tides, but the Canadian government eventually decided something needed to be done about Ripple Rock. As the CBC documents, two attempts at drilling off the mountain top failed, with fatal results, before the plan of exploding the rock was formed.

The happenings of April 5 were the culmination of 28 months of work, writes Pat Brennan for the Toronto Star, as miners tunneled under an island and then the seabed to create two vertical shafts in the mountain. As the work progressed, Brennan writes, rumors spread like crazy. A nearby town would be flattened, or a tidal wave would cause damage as far away as Japan, or the explosion would kill millions of sea creatures.

“There was even talk that the explosion would cause the big one–an earthquake that slides B.C. into the Pacific,” cameraman Bill Roozeboom, who documented the project, told Brennan.

In the midst of all this expectation, after people for miles around had been evacuated, the plunger was pushed and the country watched as pieces of Ripple Rock flew into the air in a giant plume of water. Because it was underwater, the explosion took ten times the amount of explosive material as it would have on land, the CBC reported.

The explosion blew 764,000 U.S. tons of rock and water high into the air, writes the CBC, causing high waves. “A mere handful of stunned fish were found later confused, but alive,” writes Brennan. Among the watchers were several British atomic weapons researchers sent to observe. They wrote up their thoughts in a letter to Nature.

Ripple Mountain ended up being shorter. Its peak is now 46 feet under the surface, making the passage safer for ships. But, writes Brennan, "the 20 or more cruise ships that sail past Campbell River each summer Day still leave Vancouver and Seattle at precise times, so that they arrive at Seymour Narrows at slack tide. Even they can't handle the fast currents that still rip through the narrow passage."