NASA's Curiosity rover recently captured a picture of Mars that shows a cleanly cut "doorway" into a rockface.
Space enthusiasts and conspiracy theorists have argued over what could be inside the doorway or if it is just a natural feature in the rock.
The grainy image taken by the explorer's Mast Camera on Sol 3466 (7 May) appears to show an opening with filed away rock.
The American space agency has not provided an official explanation for the doorway, but it has sparked online debate about what it depicts.
"A formation on Mars that appears to be a portal and a wall nearby that looks artificial," one Twitter user wrote.
"It's obviously not a little door, it's just a flat piece of broken rock," one Reddit user stated.
The discovery comes as the US Congress prepares to hold its first open hearing on UFOs in over 50 years.