Europa Monolith

The Europa Monolith, or Great Wall, is a monolith on Jupiter’s moon Europa, in the Arthur C. Clarke Space Odyssey series.

Great Wall
The Great Wall is the name given to the monolith on the surface of post-Lucifer Europa, near the Europan community called Tsienville. It is the largest known monolith, approximately 10 kilometers high. Unlike other known monoliths, the Great Wall stands horizontally, rather than vertically. It is not given a "TMA" title. Like known Monoliths, the Great Wall follows the 1:4:9 dimensional ratio and has a jet black color. However, its exact size varies throughout the Space Odyssey series.

2010: Odyssey Two
The epilogue (taking place in 20001 AD) of 2010: Odyssey Two alludes to the Great Wall, although the Monolith's name is not explicitly given. Standing on the border of day and night on Europa, this form of the Great Wall is considerably large, ten times larger than the tallest aliens living on Europa.

2061: Odyssey Three
The Great Wall makes its first confirmed appearance in 2061. The gigantic Great Wall, measuring about 2 kilometers on its longest axis, is lying on its side, giving itself the impression of being a wall. It is later revealed that the Great Wall assumed this position after being knocked over by a diamond meteorite, which damaged the immense Monolith.

3001: The Final Odyssey
The Great Wall makes its final appearance in 3001. Like its 2061 incarnation, the Great Wall in 3001 stands horizontally. Its size, however, has been upped to more than 20 kilometers on its longest side, making it the largest Monolith by far.