No airport in the world has been the subject of such persistent conspiracy theories as Denver International, which celebrates its 20th birthday today.

Soon after the first flight – United’s service to Kansas City – left the tarmac, the murmurings began. They initially focused on the view of the airport from above.

Why, asked some, does it so uncannily resemble a swastika? Surely this is evidence of some evil right-wing plot?

Or just a coincidence? Many airports, particularly those with four or more runways, must look like this from above? Well, not really.

JFK looks like this:

Las Vegas International looks like this:

This is Miami:

Couldn’t designers have chosen a less controversial layout? It's not as if they were struggling for room – it’s practically surrounded by desert. The sort of desert you’d expect to see UFOs hovering over.

But there’s more. Conspiracy theorists soon turned their attention to the airport’s interior. Over the years Denver International has made strides to fill its terminal building, and the surrounding area, with artwork. There was Mustang, by El Paso artist Luis Jiménez. The giant sculpture actually killed poor Jiménez in 2006, two years before Denver adopted it. A section of it fell on him, severing an artery in his leg.

The terminal building itself is a work of art.

But it was a collection of bizarre murals by Leo Tanguma, now removed, that really generated interest.

With names like "In Peace and Harmony With Nature" and "The Children of the World Dream of Peace", they depicted – the artist said – the triumph of environmentalism and peace over war and destruction.

Or, according to some, the true agenda of the "New World Order".

A typical analysis of the seemingly innocent mural above reads: "Children of all colours, dressed in folkloric costumes give weapons wrapped in their country’s national flag to a… German boy? Huh? Yes the Bavarian costume leaves no doubt. The boy at the center of the image, holding the hammer and apparently building something is German. Even the American kid (dressed as a boyscout) seems eager to give his weapons and flag to the German boy.

"You’re in the largest airport of America, in the middle of the USA, and this is the mural we display. America joyfully submitting to Germany. It’s just too odd to compute. This, it would seem, represents countries of the world giving up their military might and their national identity for ‘the common good’. Another reference to a New World order, with one government and one army."

They also point to other “masonic” symbols, such as this stone (pictured below), which features in the airport’s “Great Hall”. The New World Airport Commission, eh? No such organisation exists, people were eager to point out. Yet.

The presence of gargoyles is also said to demonstrate that something “evil” is lurking.

So what is the true purpose of Denver International Airport, if not to usher in travellers to the state of Colorado? Some conspiracy theorists would claim it is a giant military base, or even a concentration camp, just waiting to be used to assist in the enslavement of the American people.

They point to the high cost of the airport - $4.8 billion . That’s nearly $2bn over budget. Never mind that Hong Kong International Airport, completed just three years later, cost $20bn. Or that Dubai International Airport earmarked $82bn for expansion back in 2007.

They point to the amount of land used – 34,000 acres, or 140km2 - more than any other US airport. OK, King Fahd International Airport covers 780km2, but surely that just means it too is earmarked for use as giant prison.

There’s also vague references to a vast underground facility, that may or may not connect to other deep subterranean military bunkers throughout the country.

According to the website VigilanteCitizen.com: “Analysis of the data available makes me reach at least one conclusion: this gigantic structure will eventually become much more than a regular commercial airport. It has the capacity to handle a huge amount of people and vehicles, leading observers to think that the structure might be used as military base and others even add that it will be used as a civilian concentration camp in the near future.”

Scary, no?

So what does the airport say? It declined to answer Telegraph Travel’s queries (as well it might), but Robert Blaskiewicz of the Sceptical Inquirer managed to pin down Matt Chasansky, the Art Program Manager, and Jenny Schiavone from the airport’s media team, back in 2012.

“You name a conspiracy theory and somehow we seem to be connected to it,” said Chasansky. “But probably the most common is that there’s an underground city and that it is a part of a network of underground cities that the government or some sort of shadow international government, or aliens, are building, depending on your perspective... or Masons...

“I can’t give a tour in the airport without at least one person attaching themselves to the tour and starting to ask questions about it. There’s not [any] evidence you can provide, there are no assurances you can give that the conspiracy theory is wrong, because ... obviously it’s going to be a cover up or brainwashing or chips installed in brains.”

Cover-up or common sense? Make up your own mind.

Update - March 4, 2015

Following the publication of this article, a spokeswoman for Denver Airport did respond to Telegraph Travel.

Firstly, there's method in the "swastika" runway madness. "DIA's six runways do not cross and can be used simultaneously in any weather condition," she said. "All of DIA's runways support the largest jets currently flying. We think the shape looks like a pinwheel."

And the New World Airport Commission?

"[It's] actually the New—World Airport Commission," according to the airport. "It was designed by a planning and advocacy group consisting of local business and political leaders. The group had absolutely no association with the 'New World Order'."