Named 'Marshenge', alien enthusiasts have been comparing it to the famous stone circle at Stonehenge in Wiltshire, England.
It was originally imaged by by the Mars Reconnaissance Orbiter on September 24, 2012 at 3:29 p.m. local time in Nilosyrtis Mensae (28.064°N, 75.956°E).
The original-image can be downloaded here - watch out its a .jp2 of 681mb
hirise-pds.lpl.arizona.edu/download/PDS/RDR/ESP/ORB_028800_028899/ESP_028891_2085/ESP_028891_2085_RED.JP2
This formation is impressive with Marshenge-hill being approximately 359 pixels in diameter, and the stone “circle” is about 188 px, translating to 90 meters and 47 meters respectively.