At first glance, Seymour Narrows looks like a reasonably serene channel—a 5 kilometer span of blue connecting Vancouver and the Georgia Strait in the south to the tangled network of inlets and passages further north along the British Columbia coast. A safe passage.
On closer inspection, however, its inner turbulence becomes apparent: ominous whorls and upwellings, the hydraulic chasms of frothing eddy-lines and whirlpools, and, of course, the sheer velocity of the ocean as it not-so-gracefully slips between Vancouver Island and the Discovery Islands at around 8 meters per-second. Rescues aren't all that unusual.
There are a lot of channels and cuts like Seymour in the long coastal maze between Seattle, Vancouver, and, eventually, Alaska known as the Inside Passage. The Passage is a popular waterway, offering a linkage of shortcuts protected from the excesses of the open Pacific Ocean. But it also comes with its own threat in the form of tidal currents. These oceanic capillaries are where the sea breathes in and out in the form of tides. Rapids like Seymour's are the result of a differential between tidal elevations at either end. These differentials essentially create bi-directional ocean-rivers.
Until 1958, Seymour Narrows had a special hazard in the form of Ripple Rock, a pair of towering rock peaks topping out just a few feet underwater. The rocks themselves and-or the resulting standing waves could really fuck up a boat. Beginning with the steamship Saranac in 1875, 120 vessels were sunk or damaged by the hazard, in wrecks claiming 114 lives in all.
Then, in 1958, the Canadian government decided it was time to take action. In one of the first coast-to-coast live television events in Canadian history, the peaks of the hidden rock, which had been tunneled into from underneath the seabed, were packed with 1,270 metric tons of Nitramex 2H and blown to dust (and mud). It's been claimed to be the largest non-nuclear planned explosion in history, at least at the time.