For the past 10 years, NASA's Curiosity rover has been trundling around the surface of Mars, taking photos in its quest to understand the history and geology of the red planet and perhaps even find signs of life.
Last week it took a photo which appeared to show a doorway carved into the rock. It's the sort of thing that on Earth might indicate an underground bunker, such as an air-raid shelter.
At first sight, the picture is totally convincing. At second sight, maybe not. The passage seems to go in only a short way before the steeply descending roof meets the floor.
And then those killjoys at NASA tell us its only about 45 cm high. Still, who said Martians had to be the same height as us? But then geologists point out several straight-line fractures can be seen in this site, and the "doorway" is where they happen to intersect.
Such a pity. It would have been so exciting if it had been a real doorway. Instead it joins the face on Mars, the spoon on Mars, the cube on the Moon, and all the other things seen in photos from space that turn out not to be as exciting as we thought.
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