An image posted at Mars Anomaly Research(1) shows what appears to be a human skull lying on the ground of the Red Planet. According to Joseph P. Skipper's report, the anomalous formation "looks incredibly very much like an anatomically correct humanoid skull." Skipper has created a series of photo enhancements to support his argument. He also mentions the image, taken by NASA's Spirit rover, may have been tampered with. Read his full report here(2).

1. http://www.marsanomalyresearch.com/evidence-reports/2006/102/mars-humanoid-skull.htm
2. http://www.marsanomalyresearch.com/evidence-reports/2006/102/mars-humanoid-skull.htm