Russia's Nuclear Sub Nightmare is the 29th episode of Seconds from Disaster and explains what knocked nuclear sub Kursk.
During a training mission, a leaking torpedo on board the Kursk leaks hydrogen peroxide. It reacts with rust in the submarine and blasts the front of the Kursk. Although the submarine had explosion proof walls, the explosion spread through the ventilation shafts. 135 seconds later, another explosion rocks the submarine. Then it sinks, killing all but 23 men on board. While they wait to be rescued, special boards that produce oxygen to breathe drop into the oily floor, starting a reaction that creates a fire, killing the survivors.