Inside plane that if spotted could mean you have only five minutes left to live

For the plane-spotters among you, if you look up to the skies and see a certain aircraft it usually means important things are going on over your head.

In the worst case scenario it means you have about five minutes left to live, though provided you have some sort of breaking news app, you'll probably hear about it from there first.

This is the E-4B, sometimes known as the 'doomsday plane', which is a military version of the Boeing 747 Jumbo Jet designed to be used as a flying nuclear bunker.

If s**t goes down in an apocalyptic manner and US officials who are supposed to co-ordinate some kind of response are too far from shelter, then the hope is they can get into these planes - of which the US has four - and run the response from the skies.

If you spot this plane flying around it could be a sign that nuclear war is on the way, though before you start panicking too much, it also serves as a transport for the Secretary of Defense, follows Air Force One around and can provide support on other missions.

This plane is designed to serve as a flying nuclear bunker. (US Department of Defense/CNBC)

Just because this thing is in the sky it doesn't mean the world is about to end, though if it did then you'd probably want to be on board one of these things.

Back in 2013, people were invited on board one of the E-4Bs to have a look around and see what it's like on the inside, and it's packed with all sorts of things you'd need to survive above a nuclear war.

According to CNET, at least one of the four planes is always ready to take off within minutes and is capable of having a crew of up to 112 people, while the aircraft themselves are jam packed with communication technology.

The aircraft is packed full of briefing rooms, communication stations and even accommodation in case the people on board the flight need to stay up there for a while.

While the E-4B can fly for around 12 hours at 600mph it can also refuel without landing.

In the event of disaster officials would be able to get on board the plane and still give orders, whether anyone would hear them is another matter. (US Department of Defense/CNBC)

While the inside of the plane is packed full of equipment needed to co-ordinate a disaster response, the exterior is designed to make sure the occupants survive whatever comes their way.

It contains thermal and nuclear shielding to keep out the damage a nuclear disaster would pose to the crew, as there's no point building a flying nuclear bunker if it doesn't protect the people inside.

Fortunately, humanity has made it almost 80 years since the invention of the atomic bomb without the world spilling into all out nuclear war and we're all crossing our fingers to make sure it doesn't happen in the future.

Seeing one of those 'doomsday planes' flying above you doesn't mean the nukes have been launched, it's far more likely that the aircraft is carrying out one of its many other duties not triggered by the end of the world as we know it.