It's a natural feature of the rock, NASA's Ashwin Vasavada told Gizmodo, but of course he would say that about the beckoning alien tomb on Mars observed by Curiosity Rover. John Hurt, of course, is sadly unavailable for comment.
"It's just the space between two fractures in a rock," Ashwin Vasavada told Gizmodo in a phone call today. Vasavada is a project scientist in the Mars Science Laboratory, and he said the formation is definitely not the entrance to a video game's dungeon level. "We've been traversing through an area that has formed from ancient sand dunes," he said. These sand dunes were cemented together over time, creating the sandstone outcrops Curiosity is passing by.