Extraterrestrial theorists believe they've found proof of alien life on Mars.

YouTube channel Paranormal Crucible claims NASA images show a 2km long downed "alien mothership" on the surface of Mars.

The images from the video were found in footage taken from NASA's Mars Global Surveyor - which surveyed the red planet until 2006 when the space agency lost contact with it.

"I have processed, colorised and rebuilt the object and in my opinion it is a craft of some kind, possibly tens of thousands of years old," explains the description underneath the video.

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"It's also over a mile long, so this must have been a mothership, either a Martian one or possibly an off planet alien species, but why did it crash?"

ALIEN LIFE? Conspiracy theorists believe this is proof of an alien ship on Mars. Photo / Paranormal Crucible / YouTube
ALIEN LIFE? Conspiracy theorists believe this is proof of an alien ship on Mars. Photo / Paranormal Crucible / YouTube

But some people who watched it appear not to be convinced and have their own theories about what the object is.

One said: "It's sunken ground, not raised, look at the location of the light and the shadows compared to the surrounding craters."

Another dismissed claims of alien life, believing it's a deep ditch but questioned "how the hell it got there... It's in the middle of a very flat desert".

This is what the ship could have looked like. Photo / YouTube / Paranormal Crucible
This is what the ship could have looked like. Photo / YouTube / Paranormal Crucible

It's not the first time conspiracy theorists have claimed an alien civilisation is living on the red planet.

Last year, astonishing photos taken by NASA appear to show a "pair of wheels on the side of a hill" which so-called "truth-seekers" believe is evidence aliens used "advanced forms of transportation" up there.

Alien life forms would have a tough time surviving, as the planet has no liquid water on it and often reaches temperatures as low as -125C.