standard rugby union pitch, but what's it  like to be inside, and how does it work?

The Dmitriy Donskoy is far narrower  than it is long, at 25 metres,  

but don't be fooled by its streamlined  design. This submarine can displace 48,000  

tonnes of water - that's over 19 Olympic  sized swimming pools filled to the brim.

While it can carry a crew of 160 men at a  maximum speed of 27 knots when submerged,  

or 22 knots at the surface, it's not built for  traversing the world's oceans. Instead, it's built  

to sit in hostile environments for months - think  the Arctic Circle and you're on the right tracks.

Power comes from a pair of OK-650 reactors,  

which, unlike your car that almost  certainly uses an engine or motor,  

relies on splitting atoms' nuclei. Fuel comes  from 21-45% enriched uranium-235, and each  

reactor produces 190 megawatts of thermal power.  With a solar panel producing around 200 watts,  

that's about 1.9 million solar panels'  worth of power, assuming it's a sunny day.  

For car fans, that's around 50,000 horsepower, or  more than 300 modestly powered family hatchbacks.

It was first commissioned in 1980 and is the  only remaining submarine of its type in service,  

although two still sit on the side as reserves.  Another three have been retired, while the seventh  

of its type never made it to production. This  single submarine is the oldest of the bunch,  

with the others being built over the next decade.  Its purpose was to defeat enemies during the Cold  

War; an answer to America's Ohio-class submarine,  known locally as the Akula, which means shark.

The average ocean depth is over 3,600 metres, and  the deepest section in the Mariana Trench is over  

11,000 metres deep. You might be surprised that  this submarine is only tested to 900 metres, then,  

but that's with good reason. Light struggles to  reach these depths, making it almost invisible to  

spot, but the pressure is over nine megapascals,  or around 90 times that of the surface.

In 1990 Russia began re-work on this giant  piece of engineering in a process that took  

over a decade. It wasn't until 2002 that  it was relaunched, almost entirely rebuilt,  

bringing it in line with modern standards  and fitting it with the latest hardware.

It's capable of launching ballistic missiles  anywhere in the world, although this is very much  

only used for practice purposes. There are no less  than 20 launchers onboard for the RSM-56 Bulava  

ballistic missile, which may sound like a bunch  of jargon, but let me break that down for you.

Each missile measures around 12 metres  in length and two metres in diameter,  

weighing over 36 tonnes. It's good for  targets over 8,000 kilometres away.  

In other words, it can almost  scale the North Pacific Ocean,  

reaching the coastline of Japan from  the Western sands of California.

Other armament includes four 21-inch  torpedo tubes and two 26-inch torpedo tubes.

To keep everything moving, the crews have  completed many tests since its relaunch.  

27 September 2005 saw the first Bulava missile  test from the surface of the White Sea off the  

North coast of Russia near to Finland. Its first  underwater firing followed on 21 December 2005,  

on the Kera Test Range on the far side of Russia.

A further, and this time failed, test in 2009  resulted in a spiralling blue light which was  

spotted from Norway and Sweden. Captured  in photographs, the people lucky enough to  

have seen this said it looked like a blue  light emanating from behind the mountains,  

stopping mid-air before spiralling  outwards. Lasting around 10 minutes,  

and covering a large area of northern Norway,  one of the country's famous astronomers had  

initially believed it to have been a never  before seen variant of the Northern Lights.  

2010 saw, once again, successful attempts at  shooting targets back at the Kera Test Range.

While crews are trained to deal  with immensely powerful weapons,  

they're also expected to live inside the submarine  for long spells of time. After all, it's capable  

of staying submerged for 120 days if not more -  that's around four months. You might not expect  

such a utilitarian vessel to be so lavishly  equipped, but it's no hostile environment.

Nicknamed the floating Hilton, because  accommodation is so good, crews can expect  

similar styles of luxury to ships that occupy  the water's surface. Wooden panels line many of  

the walls, and plush padded chairs take the  place of hard-wearing plastic alternatives.  

The doorways are even fully sized, which  is a real luxury in the military world.

It doesn't stop there, though. On rest days,  

or even on workdays, the crew can get a  full workout in, using the onboard gym,  

before taking a dip in the submarine's indoor  swimming pool - yes, an underwater swimming  

pool! It might only be around 2 feet deep, but  the pool is designed to help relax the workers,  

and to provide them with a sense of familiarity.  This is all before heading off to the sauna.

But this is soon to become history, as  the typhoon class submarine makes way for  

the newer, and smaller, Borei class. What  this means is that Russia's typhoon will  

likely hold the title for the world's  largest submarine for many years yet,  

even beyond its retirement which is set  to be sometime during or after 2026.

Looking toward the future, plans have  been submitted for another Russian-built  

submarine that would absolutely dwarf the  typhoon class submarine by comparison.  

Twice the length, the suggested vessel  is not intended to be a military vehicle,  

but rather a transportation device for  liquid natural gas under the Arctic ice.

There's no word on the  possibility, or feasibility,  

of this yet, but one thing's for sure.  It would be almost immune to piracy,  

making it one of the safest ways to carry  expensive resources around the world.

If you had the chance, would you venture into  the deep seas in a submarine like the typhoon  

class Dmitriy Donskoy, or do you prefer to  keep your feet safely grounded on terra firma?  

Regardless, with or without  its history as a military  

vessel that has the capability of  inflicting huge amounts of damage,  

the world's largest submarine is a sight to  behold, and an extraordinary feat of engineering.