Ganymede

The final frame received as the third mission approached the surface. I will not speculate here on the exact nature of the curiously regular feature seen in this image: those interested can find extensive, if somewhat feverish, speculation in the forums. Ignore the poster known as 100suns: it is believed to be a partly-trained AI making superficially-plausible-but-bizarre inferences from inadequate data. It is not, as it claims to be, party to insider knowledge. The 'photographs' are not real.