Hale crater

Hale is an impact crater on Mars, named after the American astronomer (no relation to the discoverer of the Hale-Bopp comet). It is located at 35.7°S, 323.4°E.

After the European Space Agency's Mars Express orbiter made some pictures of the crater in 2004, some... people decided that the images are evidence for a Martian civilization. The claim is based on the pattern of regular features seen in an ESA publicity image of the crater, which conspiracy theorists interpret as, well, anything from crop fields to buildings. Unfortunately, the rectangular pattern is caused by image compression artefacts.

The image that is most usually cited as evidence of the claim is not a direct photograph of the surface, but a computer-generated image: it's a computer 3D-graphics rendering showing a "perspective view" of the crater, where the underlying 3D model is elevation data reconstructed from stereographic images and the texture mapped onto it is an image of the surface. Thus, the compression artefacts in the original image got propagated to the several renderings of the 3D model and were rotated and distorted in the process. And yes, the HRSC camera does compress its images to transmit them to Earth:

Similar artefacts can be observed in Google Earth's images of areas with low contrast, for example this image of a river.