NASA's Curiosity rover has been exploring Mars' surface for the past 10 years while taking pictures in an effort to learn more about the planet's geology and history and potentially even look for evidence of life. It captured a picture last week that appeared to depict a gateway chiselled into the rock. It's the kind of thing that on Earth may point to an air-raid shelter or other underground bunker. 

The picture is completely believable at first glance. On second thought, perhaps not. Before the sharply sloping roof hits the floor, it appears that the corridor only extends a short distance. Then those pessimists at NASA inform us that it is only 45 cm high. Who, however, mandated that Martians be of an equal height as humans? 

Geologists then point out that this location contains many straight-line fractures, and the 'doorway' is where these fractures happen to intersect. What a shame. If that had been a real doorway, it would have been so thrilling. And to make matters worse, the 'doorway' now joins the even longer collection of bizarre pictures, such as the cornflake that resembles Australia, the kittens that resemble Hitler, and so forth. 

Who hasn't witnessed a face in the clouds, too? Humans unfortunately attempt to transform ambiguous or unfamiliar images into objects that resemble those they are familiar with. Pareidolia is the name given by scientists to this tendency. It's simple to comprehend why it occurs. 

We most likely developed this propensity because it offered us an advantage to be able to see crucial objects, such as predators or faces, even in dim light or when they are partially veiled. Additionally, false positives—seeing a predator where none exists—are preferable to going unnoticed and getting eaten. 

Conspiracy theorists who claim that the doorway is true proof of Martian life and that scientists are covering it up won't be dissuaded by plausible answers. The pictures wouldn't be being made public if I were attempting to hide something up! So it doesn't seem like a conspiracy is very likely. However, there is a lesson for serious extraterrestrial life researchers as well. 

Remarkable claims call for equally extraordinary evidence, as noted by astronomer Carl Sagan. In accordance with this axiom, researchers hunting for proof of extraterrestrial life require considerably stronger proof than, say, someone looking for a geological structure. 

And despite decades of looking, scientists have not discovered any proof of life on Mars. There is still a chance that Mars ever supported life. We might still come upon some preserved remains of early cellular life. However, it seems odd to locate an artefact like a spoon or a doorway all of a sudden.