Beavers built a dam 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 1975, as it's visible in arial photographs. (Many from space).
So 50+ years for rodents maintaining a project seems pretty neat.
Yeah after we slaughtered them to near extinction, it sounds like they are making a good comeback. (Fur hats and anal gland perfume have different yearning these days)