A cleanly cut 'doorway' which can be seen in a new photograph of Mars released by Nasa is sparking alien conspiracy theories online.
Space fans and conspiracy theorists alike have been united in their eagerness to discuss exactly what the 'door' could lead to, or whether it's just a natural part of the rock.
The grainy image, which was taken by Nasa's Curiosity rover on its Mast Camera on May 7, appears to show a doorway cut into the surface of the rock, which one social media user described as "a portal."
It is unclear if it goes deeper into the rock.
So far, NASA has not given an official explanation of what the photograph shows, but some internet users have let their imaginations run wild.
One said: "there's at least five martians camouflaged in there."The 'doorway' may look big enough for a person to walk through, but it could only be a few centimetres tall in reality.
This didn't stop one Reddit user saying: "So what if it’s only a few inches? how do we know what size our overlords will actually be?"
But other users were sceptical, warning it could have been caused by a natural stress fracture due to seismic activities.
"I would really like and believe that that's an alien door. But sadly I think its not," one commented on Reddit.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To know
UFO conspiracy debunker UFO of Interest tweeted that looking at a mosaic view of the images from Curiosity's camera showed that instead of being an "entrance to a secret underground tunnel," the photo showed a "little niche" in the rock.
The tweet added there were other blocks, fractures, shapes and other erosive features all over the rock face.
British science writer Mick West, who investigates and debunks pseudoscientific claims and conspiracy theories, tweeted that the 'doorway' was actually "just a gap in a fractured rock outcrop."
On Thursday, scientists provided the first glimpse at a humungous black hole that resides at the centre of our galaxy.
Dubbed Sagittarius A*, astronomers revealed the black hole is four million times the mass of our own sun.