There is a huge ‘monolith’ on Phobos, one of Mars’s moons

Many people are vexed by the Phobos monolith. It has inspired all manner of alien-based conspiracy theories and this fascinating discussion on Reddit. It even gave its name to an album released this year by Les Claypool and Sean Lennon Ono. So what is it? Image: Mars Global Surveyor/Nasa

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