Disturbing reason why seeing this plane flying above you could mean you have just minutes left to live

If you see a plane two-thirds the size of an American football field and as tall as a five-story building soaring through the sky, it could mean one of two things.

There are two scenarios in which you'd spot this beast of a plane somehow flying over your head. The first is simple: US government officials are using the huge militarized version of the Boeing 747-200 to get from A to B.

The E-4B is the US Air Force's militarized version of a Boeing 747 and it towers 63.5 feet above members of the US Air Combat Command Security Forces guarding it during a trip to South Korea in 2013 (Jacquelyn Martin-Pool/Getty Images)

The second is a little more chilling and I'll let the US Air Force explain what its aircraft, dubbed the E-4B, is purposed for.

Its mission statement reads: "The E-4B 'Nightwatch' serves as the National Airborne Operations Center and is a key component of the National Military Command System for the President, the Secretary of Defense and the Joint Chiefs of Staff.

"In case of national emergency or destruction of ground command and control centers, the aircraft provides a highly survivable command, control and communications center to direct U.S. forces, execute emergency war orders, and coordinate actions by civil authorities.

"The conduct of E-4B operations encompasses all phases of the threat spectrum. Additionally, the E-4B provides outside the continental United States travel support for the Secretary of Defense and his staff to ensure Title 10 command and control connectivity."

An air-to-air right side view of an E-4B advanced airborne national command post aircraft being refueled from a KC-135 Stratotanker aircraft (USAF)

To put it in black and white, it's in the event the White House and the Pentagon are nuked. I mean, Washington, D.C. would probably be a target for two dictators we've come to know rather well in recent years - North Korea's Kim Jong Un and Russia's Vladimir Putin.

Although there are four of these planes, each of them is stored at the Offutt Air Force Base, Nebraska, which is home to the US Strategic Command.

So what makes these planes so good that they can withhold nuclear blasts then?

Let's start with the sheer size of them. They are 231 feet long, 63 feet high and have a wingspan of 195 feet, while they weigh 800,000 pounds - that's the equivalent of flying 80 African elephants through the sky.

They've got four electric turbofan engines that each pack a punch - the thrust power being 52,000 pounds for each engine - so 208,000 pounds in total.

They are also kitted out with nuclear and thermal effects shielding and an air-conditioning system for cooling electrical components, as well as the capability to refuel in-flight and can fly for 12 hours straight before needing to.

They cost a staggering $223.2 million, and one of the four planes is always on alert 24 hours a day 'to provide direct support to the President, the Secretary of Defense, and the JCS' (Joint Chiefs of Staff).