Beavers built a damn in Alberta (850 meters) that is twice the length of the Hoover Dam. It isn't the technical marvel as the Hoover Dam, but one would have to be lying to say it's not impressive.
Extra info: we know it has existed since at least existed in 1975, as it's visible in arial photographs. (Many from space).
So 50+ years for rodents maintaining a project seems pretty neat.






