The Seymour Narrows is a 3-mile/5 km stretch of the Discovery Channel north of Vancouver Island, British Columbia that is notorious for the strength of the tidal currents flowing through it. The average width of the narrows is just 750 meters. During extreme tides, the current through the narrows is subject to severe Venturi effect, resulting in an increased velocity that can reach 15 knots. For much of its modern history, there was an additional hazard in the narrows called Ripple Rock, a shallow obstruction that claimed no fewer than 119 ships and 114 lives. In 1958, after months of tunneling and preparation, Ripple Rock was blown up in the largest commercial, non-nuclear explosion ever recorded in North America. Still, the navigation of Seymour Narrows is dependent on tidal and other conditions, and requires skill and technical accomplishment.