K-141 Kursk

Wrecked hull of Kursk after it was raised a year later.

hull of Kursk after it was raised a year later.

Here is,
according to Wikipedia, what is believed to have happened to the Kursk:

Kursk submarine disaster occurred during a major Russian naval exercise in the
Barents Sea on Saturday, 12 August 2000. The Kursk, an Oscar-class submarine
(Russian: Project 949A Антей), was preparing to load a dummy 65-76 “Kit”
torpedo when a fire, followed by a large explosion, caused the ship to sink.
Nearby ships registered the explosion but did not know what to make of it. A
second, much larger, explosion took place two minutes and 15 seconds later, and
was powerful enough to register on seismographs as far away as Alaska.

Russian Navy did not recognise that the vessel had sunk for more than six hours
and because the emergency rescue buoy had been intentionally disabled, it took
more than 16 hours for them to locate the sunken ship. Over four days they used
four different diving bells and submersibles to try to attach to the escape
hatch without success. The navy’s response was criticized as slow and inept.
The government initially misled the public and media about the timing of the
accident, stating that communication had been established and that a rescue
effort was under way, and refused help from other governments. The Russian Navy
offered a variety of reasons for the sub’s sinking, including blaming the
accident on a collision with a NATO vessel. On the fifth day, the Russians
accepted British and Norwegian offers of assistance. Seven days after the
submarine went down, Norwegian divers finally opened a hatch to the rescue tube
in the ship’s ninth compartment, hoping to locate survivors, but found it

Kursk was an Oscar-class submarine, twice the length of a 747 jumbo jet, and one of the largest submarines in the Russian Navy.

was an Oscar-class submarine, twice the length of a 747 jumbo jet, and one of
the largest submarines in the Russian Navy.

official investigation after most of the wreck was raised along with analysis
of pieces of debris concluded that a faulty weld in the casing of the practice
torpedo caused high-test peroxide (HTP) to leak, which caused the kerosene fuel
to explode. The initial explosion destroyed the torpedo room, severely damaged
the control room, incapacitated or killed the control room crew, and caused the
submarine to sink. The fire resulting from this explosion in turn triggered the
detonation of between five and seven torpedo warheads after the submarine had
struck bottom. This second explosion was equivalent to between 2 to 3 tonnes
(2.0 to 3.0 long tons; 2.2 to 3.3 short tons) of TNT. It collapsed the first
three compartments and all the decks, and destroyed compartments four and five,
killing everyone forward of the nuclear reactor compartment. An alternative
explanation offered by critics suggested that the crew was not familiar with
nor trained on firing HTP torpedoes and had unknowingly followed preparation
and firing instructions intended for a very different type of torpedo. Combined
with poor oversight and incomplete inspections, the sailors initiated a set of
events that led to the explosion.

was later determined that 23 sailors in the sixth through ninth compartments
survived the two explosions and took refuge in the ninth compartment. They
survived more than six hours before an oxygen cartridge contacted the oily sea
water, triggering an explosion and flash fire that consumed the remaining
oxygen. All 118 sailors and officers—111 crew members, five officers from 7th
SSGN Division Headquarters, and two design engineers—aboard the Kursk died. The
following year, a Dutch team was contracted by the Russians to raise the hull.
Employing newly developed lifting technologies, they recovered all but the bow
of the vessel, including the remains of 115 sailors, who were buried in Russia.

It is not
fully known what truly happened to the Kursk.  Many theorize that it could
have been brought down by a collision with an American ship in the area as

Below, you
will find the records uncovered via the Freedom of Information Act (FOIA).

Declassified Documents

Central Intelligence Agency (CIA)

Declassified KURSK Documents, Released June 2016
 [17 Pages, 2.3MB ]

Department of the Navy

of the Navy Declassified Maritime Reports
[16 Pages, 0.8MB ] – This,
according to the NAVY, is all the material they currently have about the sinking
of the Kursk.  These records were declassified in January if 2015.