It was long-believed that the Great Wall of China was the only man-made creation that could be seen from space, but that’s simply not true. Viewed from above, the wall looks similar to a road, and is no more significant from space than other roads around the world.
However, some beavers may have pulled it off. According to an article on Treehugger (and originally found by Jean Thie in the Google Earth Community), a massive beaver dam in Alberta, Canada can be seen from space. At 2,790 feet long (more than twice the size of the Hoover Dam), this beaver dam is quite impressive.
While it’s possible it could be viewed from space, it’d probably be tough to pick out. A small brown blob from that altitude could really blend in pretty well. Still, it’s a very impressive bit of engineering by those beavers. You can read more about it on Treehugger, or fly there yourself using this KML file.