An underground pyramid giving out gigantic amounts of energy is being “covered up” by the US government, a documentary has revealed.

In May 1992, a one megaton nuclear underground explosion in China sent shockwaves throughout the world.

The explosion was recorded by US government seismometers and their findings revealed unexpected geological anomalies in the north of Anchorage, Alaska.

Journalist Linda Moulton Howe was asked to look into the readings by counterintelligence officer Douglas Mutchler.

History Channel's documentary Ancient Aliens revealed how Linda was then contacted by an ex-naval worker – who made a startling claim.

The man – who wished to remain anonymous – claimed his father was an engineer and had a first-hand look at a secret pyramid underground.

Linda said: “His father had a Navy career and then was asked to continue to work in Alaska on a very special project that involves something underground that put out a lot of energy.”

The investigative journalist also added that there were “mining shaft elevators” in the region.

“He would go down to the base of a huge pyramid using the elevators," she claimed.

“His work was to try to understand how this was giving off what they estimated as a gigantic amount of energy.

Underground structure: Is the underground structure being 'cover up'? (Pic: NC)

“But he never would talk, because this was a government secret.”

The episode then featured author and conspiracist David Wilcock, who claimed there was a "cover-up" going on.

He stated: “There's no doubt in my mind that there's a cover-up going on and something was found there in Alaska.

“I think the reason why it has been covered up is that we would have to completely rewrite the science of human origins.

Elevators: The elevators sent the engineer to the base of the underground pyramid, it's claimed (Pic: ANCIENT ALIENS / THE HISTORY CHANNEL )

“The concept that at one point there was a global industrialised civilisation that had reached a technology that in fact superior to our own because we can not easily build pyramids like this today.”

Pyramids across the globe have been the subject of fierce conspiracy theories in the past.

The question on the origins of the pyramids of Giza has often baffled experts – with the complex structures appearing to technologically advanced for ancient Egyptians.

And supposed “pyramids” found in Antarctica has sparked claims of ancient civilisations living there, in the past.

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New episodes of Ancient Aliens are on free to air channel BLAZE every Wednesday at 9pm, Freeview Channel 63