Satellite images of a mysterious rectangular shadow on the surface of Mercury was believed to be an alien "doorway" to another dimension.

In a YouTube video posted by Secureforce10, a black rectangular structure was spotted in an image of Mercury's Victoria Rupes Cliffs captured by NASA's MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) spacecraft, which conspiracy theorists believe to be an entrance of some sort.

According to the Daily Mail, the object was first discovered by Scott Waring of UFO Sightings Daily in 2012, who reported that the structure was a "doorway" that measures about 3.4 miles long used by aliens as a passageway to another dimension.

"Unless meteor cubes are pummeling planets, this seems to be made by ancient aliens," Waring said in a statement. "That is a giant doorway for giant ships to travel through."

In the new video, however, Tyler Glockner suggested that the structure might not be a doorway but rather a massive rectangular monolith towering up above the surface.

"I took a really good look, it became clear to me that this is not an entrance at all. In fact, it's something much more magnificent," Glockner said.

According to Glockner, the angle of the sun is casting shadows on the northern walls of the craters, which means that the massive rectangular shadow is being cast by a standing structure.

He said that the shadow could be from a large rectangular building, where the sun's light is hitting the top and front of it. "The back of the structure is in darkness and we have this very long shadow being cast down on the surface behind it," Glockner said. He emphasized that this is likely since Mercury is the closest planet to the sun.

Glockner said that large rectangular monoliths of the same kind were believed to have been found on other planets and moons, including the Moon, Mars, and Mars' large moon Phobos.

NASA's MESSENGER probe started to orbit Mercury in 2011 and ended in April 2015. Close inspection of Mercury will be again be performed by the second mission to the planet, the European-Japanese BepiColombo spacecraft, which is slated to launch in 2018 and will arrive on the planet in 2024.