First discovered by someone messing around on Google Earth, it's the largest piece of animal-built infrastructure on the planet, and according to some science people, it would have taken upwards of 20 years to build, and can be seen in satellite images from 1990.
Originally constructed to keep out enemies -- like China's Great Wall -- it's now a tourist attraction for humans astonished about how such a huge thing was built by such inferior creatures. The dam acts as a moat and protects the beavers from much of their land-based adversaries such as foxes and bears. In addition, the monstrous constructs also house several generations of beavers.
Nice beaver. (OK, that was the last one -- we swear.)
For reference, normal beaver dams clock in at around half the size of this one, but nonetheless, they can typically rival the Hoover Dam in length, which is pretty good for giant rats whose only method of construction is slapping things with their tails.