Nasa's Curiosity rover has discovered what looks like an "alien doorway" on Mars, sending the internet into a frenzy.

The image, which was taken on May 7 in the Mount Sharp region of Mars, went viral on social media with people claiming it was a fake. Or the entrance to an alien temple.

The straight edges, the shadows cast, and the smooth "path" sure do look convincing. And odd.

But, according to geologists, the "doorway" is not all that it seems.

The gap is much less an entrance to a Martian temple than it is the result of "natural erosion" – a chunk of rock has fallen away by natural forces, leaving the shallow, but nonetheless strangely clean-cut gap behind.

Mars Science Laboratory project scientist Ashwin Vasavada told Gizmodo the "space between two fractures in a rock" formed from "ancient sand dunes" that have been piling up over centuries.

Pressure mounts on each layer as the dunes are buried and reburied and Mars' surface shifted.

"The fractures we see in this area are generally vertical," he said, adding that this doorway-shaped structure was likely formed in two ways.

"I think what we have here (is) either two vertical fractures, where the middle piece has been removed, or one vertical fracture, and the blocks have moved apart a little bit."

In fact, there are several clues that the crevice is not an actual entrance.

For one, it is less than a metre tall, and the Curiosity rover is about 2.2 metres tall. So these architect aliens are either very short or, logically, it's just rock.

Second of all, you can actually see there's no dark interior to the alcove. It's just an alcove.

It's entirely lucky the sharp boulder-looking rock to the left is what broke away – and it smoothed the planet's surface as it rolled away.

But logic hasn't stopped the conspiracy theorists from taking the picture and running with it.

"So what if it's only a few inches? How do we know what size our overlords will actually be?" one conspiracy theorist posted on (a now deleted) Reddit thread.

The internet has certainly had fun with this intergalactic event, photoshopping mini aliens into the original image and claiming other Martians might call it home.

"If you zoom in all the way you can see Matt Damon crouching inside the doorway," another Redditor said – referencing the 2015 sci-fi blockbuster The Martian.

And this certainly isn't the first time something mysterious has set the internet into orbit.

In 2021, China's Yutu 2 rover spotted what looked like a cube on the moon, which turned out to be a rock that wasn't even vaguely cubic.

Just last week, the world marvelled at claw-like scratches (actually, fault lines) that stretched across Mars' western hemisphere. And weeks after a flying saucer was spotted on the planet.

The sighting comes as Curiosity continues its ascent of Mount Sharp, which rises approximately 5480m from the floor of Gale Crater, where it first landed in August, 2012.

The viral image was taken on a slope called Greenheugh Pediment, which is covered with "gator-back" terrain, named after its reptile scale-like appearance.

Curiosity has been navigating this particular area for months in its slow climb to the ancient mountain.

- Georgina Noack, news.com.au