From alien coffins to fossilised lizards, some believe they have found all manner of strange items in Nasa's images of Mars.
Now, one alien hunter claims to have spotted what he believes to be a bunker on the red planet, with armed people peering out of its windows.
Andre Gignac says he made the 'stunning discovery' in a photograph released by Nasa and taken by the Mars rover Opportunity.
'These were explained as being of erosional features or caused by the way the pictures were stitched together.
'Whatever the "mundane" explanation I think we are more fascinated by the idea that there are people secretly living beneath the surface of Mars and they are part of some secret space program.
'Such a project would need vast resources and would be very hard to keep covered up, yet many think this is what is happening and that these activities are being carried out in conjunction with one or more alien species.'
The latest discovery follows a spate of supposed 'sightings' of Martian civilisation.
Last year, a Maryland-based UFO hunter spotted what he believed to be a 'decorative' box on the red planet.
The 'coffin' was discovered by Will Farrar from WhatsUpintheSky37 as he trawled through a library of pictures sent back by the Mars rover Curiosity.
'This little box sure does look like a modern coffin concrete liner,' he wrote on his YouTube channel.
'As well as the stonework on the back part of the hill that looks like stairs or some left over stonework from some old civilisations constructions.'
Alien researcher Scott Waring of the UFO Sightings Daily said he believes the coffin was made from a stone-like substance.
'What would it take to get Nasa to turn the rover around and examine the contents of this box?,' he said.
But while its strong lines have captured the imagination of UFO bloggers, scientists believe sightings such as this are down to a psychological phenomenon called Pareidolia.
This describes the brain's response to seeing faces and other significant objects in random places.
It has evolved so that human eyes can spot faces within their environment, and to help them recognise friends in a crowd.
Scientists claim we also tend to use this ability to 'enrich our imagination' and recognise meaningful shapes, even when they're not there.
'I stand ready to be convinced that I have possibly hallucinated. I won't mind being told that I'm wrong,' Cignac says.
By ELLIE ZOLFAGHARIFARD, Mail Online
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