Kursk was an Oscar II class submarine, and the last submarine built in the Soviet era. Construction began in 1990, and by the time she was completed and launched in 1994, the cold war had ended.
The Oscar II class were the largest attack submarines ever built, running about 500ft long, and 60 feet wide. When viewed in cross-section, the inner hull of the Oscar II class was circular, while the outer hull was oval-shaped. This oval-shaped outer hull gave the Oscar II ships the appearance of being fat. Between the inner and outer hulls were missile tubes containing 24 anti-ship missiles, as well as air banks.
Kursk in port.
The reason Oscar Class submarines look "fat". Lots of anti-surface ship firepower.
After the collapse of the Soviet Union, the sailors of the Northern Fleet were intermittently paid, and little money was available for repair and maintenance of the fleet. Kursk only made a single patrol between her launching in 1994 and her loss in 2000. With the lack of sea-time, it is probable that her crew was not well-trained, and certainly not proficient. It's not clear whether a well-trained and proficient crew would have altered the outcome, however.
On August 12, 2000, Kursk was participating in the largest Russian Naval exercise in 9 years, following the collapse of the Soviet Union. Three other submarines, the Russian flagship battlecruiser "Petr Velikiy" (Peter the Great) were participating, along with a flotilla of smaller vessels.
Kursk was preparing to fire a dummy torpedo at the battlecruiser Petr Velikiy, when an explosion occured. The accepted theory is that the highly concentrated HTP (High-Test Peroxide) hydrogen peroxide which fueled the torpedo leaked out and reacted, causing the torpedo engine to detonate, starting a fire.
A similar explosion caused by an HTP-fuelled torpedo was responsible for the loss of HMS Sidon in 1955, after which the British Navy abandoned torpedoes propelled by peroxide.
This first explosion was recorded on the SOSUS network, and was estimated to have an explosive force of 220-550lb TNT. A second, larger explosion measuring 6000-14000 lbs of TNT occured 135 seconds after the first explosion. Kursk came to rest in just 350 feet of water.
Rescue offers were made immediately by US, British, and Norwegian teams, but these offers were rebuffed by the Russian Navy. It was believed at the time that there were no survivors from the initial explosion.
I will turn the saddest part of this sinking over to Wikipedia:
Captain Lieutenant Dmitriy Kolesnikov, one of the survivors of the first explosion, survived in the ninth compartment in the turbine room at the stern of the boat after explosions destroyed compartments 1-5. Recovery workers found notes on his body. They showed 23 sailors (out of 118 aboard) had managed to enter compartment nine after the ship sank.
There has been much debate over how long the sailors might have survived. Some point out that many potassium superoxide chemical cartridges, used to absorb carbon dioxide and chemically release oxygen to enable survival, were found used when the craft was recovered, suggesting some of the crew survived for a significant time.
Kolesnikov's last note has a time of 15:15, indicating that he and the others in the aft compartment lived at least four hours after the explosion. 32 hours after the first explosion no sound was heard (i.e. hull tapping) to signal the Russian Submarine Rescue Vehicle Priz, when it attempted to mate with the aft escape trunk.
The oxygen generator cartridges appear to have been the cause of death; a sailor appears to have accidentally brought a cartridge in contact with the sea water, causing a chemical reaction and a flash fire. The official investigation into the disaster showed some men appeared to have survived the fire by plunging under the water. Fire marks on the walls indicate the water was at waist level in the lower area at this time. However, the fire rapidly used up the remaining oxygen in the air, causing death by asphyxiation.
In July 2002, the investigation committee concluded that a technical malfunction on a single Type 65-76 "Kit" (Whale) torpedo caused the first explosion, triggering a fire in the torpedo room which, two minutes later, caused 5-7 additional torpedo warheads to detonate.
The second explosion destroyed a large section of the submarine (at least 4 of the 9 compartments) killing up to 95 of the 118 crew members and causing the submarine to sink. Around 23 crew members survived the sinking and took refuge in the ninth compartment where they died due to carbon monoxide poisoning following a fire in that compartment (between 6 and 32 hours after the sinking).
Kursk was raised from the ocean floor in a difficult and expensive recovery project. Her bow was an unrecoverable mess and was cut off, using carbide coated cables. She was re-floated and returned to Russia, where her two reactors were decomissioned, and her side missiles were removed. Those of her 118 crew that were recovered were buried with honors.