In 2000, one of the worst peacetime submarines accidents ever took place off the coast of Russia. A huge explosion sank the giant nuclear-powered submarine Kursk, killing most of its crew and stranding nearly two dozen survivors hundreds of feet underwater. An international rescue team assembled to save the sailors, but was unable to reach them in time.

Colin Firth stars in a new movie about the disaster called Kursk, which comes out this year. Here's the true story of the doomed sub.

Carrier Hunters

One of the Soviet Union’s biggest worries during the Cold War was America’s fleet of aircraft carriers. The Soviets saw American carriers as both delivery platforms capable of launching thermonuclear airstrikes against the motherland and as hunters of the USSR’s own nuclear ballistic missile fleet. The USSR spent enormous sums on weapon systems meant to hunt down American carriers in wartime.

A photograph dated 19 October 1999 of the Kursk’s commanding officer, Gennady Lyachin, saluting after a patrol in the Mediterranean Sea. Getty Images

The Antey -class submarines were one such solution. The subs, nicknamed “Oscar II” by NATO, made up a large class of nuclear-powered boats designed to to kill large ships—particularly aircraft carriers. The Oscar IIs were 508 feet long with a beam of nearly 60 feet and displaced 19,400 tons, twice as much as a destroyer. To keep up with American nuclear-powered carriers, the Soviet subs were each powered by two OK-650 nuclear reactors that together provided 97,990 shipboard horsepower. Such power gave them a top speed of 33 knots underwater.

The Oscar IIs were big because they carried big missiles. Each submarine carried 24 P-700 Granit missiles, which themselves were each the size of a small plane—33 feet long and weighing 15,400 lbs. each. The missiles had a top speed of Mach 1.6, a range of 388 miles, and used the now defunct Legenda satellite targeting system to home in on their aircraft carrier targets. A Granit could carry a 1,653-lb. conventional high explosive warhead (enough to damage a carrier) or a 500-kiloton warhead (enough to vaporize an aircraft carrier with a single hit.)

A woman adjusts a wreath at a memorial to the Kursk. Natalia Kolesnikova Getty Images

Thirteen Oscar I and Oscar II submarines were built, including K-141—also known as Kursk.

The Torpedo That Failed

The Kursk was completed in 1994 and assigned to the Russian Northern Fleet. On August 15, 2000 the Kursk was involved in a major fleet exercise, along with the aircraft carrier Admiral Kuznetsov and battlecruiser Pyotr Velikity. Kursk was fully armed with Granit missiles and torpedoes and was to make a simulated attack on Kuznetsov.

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At 11:20 AM local time, an underwater explosion rocked the exercise area, followed two minutes later by an even larger explosion. A Norwegian seismic monitoring station recorded both explosions. One Russian account claims the 28,000-ton battlecruiser Pyotr Velikiy shook from the first explosion.

Racked by explosions, Kursk sank in 354 feet of water at a 20-degree vertical angle. One of the explosions ripped a large gash in her forward bow, near the torpedo compartment. A Russian Navy board of inquiry later determined that one of the submarine’s Type 65-76A super heavyweight torpedoes had exploded, causing the gash. The explosion was likely caused by a faulty weld that failed to hold the hydrogen peroxide fuel chamber together.

Kursk’s conning tower is visible as the submarine is towed back to Roslyakovo, Russia. Getty Images

Like many torpedoes, the Type 65-76As used hydrogen peroxide as underwater fuel. The danger was that this chemical compound can become explosive if it comes in contact with organic compounds or a fire.

According to the U.S. National Library of Medicine, “Hydrogen peroxide is not itself flammable but can cause spontaneous combustion of flammable materials and continued support of the combustion because it liberates oxygen as it decomposes.” In one instance recorded by the NLM, “Leakage from drums of 35% hydrogen peroxide onto a wooden pallet caused ignition of the latter when it was moved. Combustion, though limited in area, was fierce and took some time to extinguish. Leakage of 50% peroxide onto supporting pallets under polythene sheeting led to spontaneous ignition and a fierce fire.”

The Fateful Moments

So what happened on board the Kursk? The likely chain of events was something like this: A hydrogen peroxide leak started a fire, which in turn detonated the Type 65-76A’s 900-lb. high explosive warhead. This probably started the gash in the hull above the torpedo section. The second explosion would have been the detonation of the remaining torpedoes aboard the submarine.

Lieutenant Captain Koselnikov, who penned a note two hours after the sinking of the Kursk describing 23 crew still alive. None of the crew survived to be rescued. Getty Images

The Kursk’s sinking didn’t kill all of its 118 crewmembers—at least not right away. One of the ship’s officers, Lieutenant Captain Dmitri Koselnikov left a note dated two hours after the second explosion recording 23 survivors. Despite a hastily organized rescue effort, including British and Norwegian rescue teams, the Russian government was unable to reach any of the survivors in time. The wreck of the submarine was recovered in 2001 and returned to the Russian Navy submarine shipyards at Roslyakovo.

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