Dam.

Some busy beavers in Northern Canada have built the largest beaver dam ever recorded - and they've actually been at it over 40 years.

Jean Thie found it while tracking melting permafrost in the far north. She noticed a huge structure in the middle of a nearly inaccessible part of Wood Buffalo National Park, so she got park rangers to take a closer look.

The dam is an impressive 2,800 feet long (most dams are anywhere from 30 to 300 at their longest) and sprawls out in almost a birdlike shape.

Thie says construction probably started around the mid 1970s, but that it clearly continues today.

"You can see a lot of fresh sticks. This one has grasses growing on it and it's very green."

He thinks that it probably started naturally, with beavers taking advantage of some felled trees. There's no telling how many beavers call the megadam home, but it could be hundreds.

In the wild normally only a single family of beavers would live in a dam, but that still could be as many as 18 beavers including parents and kits.

The dam is so large that satellite imagery from NASA actually shows the dam back in the 1990s.

Beavers build dams to create pools of water to ward off predators and to float food and building materials. Thie says that since the region is very flat beavers need to make larger structures to have an effect.

Thie says there are 2 more large dams in the area that might merge with this one, creating a megadam.

The world's previous largest dam was in Montana and was about 2,100 feet.