The largest structure built by an animal is that of a beaver’s lodge and dam. Beavers (Castor canadensis) of North America use mud, wood, vegetation and stones to dam up water to form a pond and then build a lodge for winter refuge. The largest lodge ever recorded was 12.1 m (40 ft) across and 4.8 m (16 ft) high and the longest beaver dam ever recorded measured 1.5 km (0.9 miles) long.

Lodges typically measure 3-4 m (9 ft 10 in -13 ft) across, 3 m (9 ft 10 in) high, and take 20 days to build using 3 tons (6,613 lb) of materials. On average, a family of 4 beavers can build 1.5 m (4 ft 11 in) of dam wall per day. The design of the dam usually reflects the rate of the water flow, i.e the faster the water flows, the more curved the dam will be. Once the pond has been created, canals are excavated for the beavers to reach safely into the wooded areas (to collect more wood) by water and not on land where predators dwell. The longest documented canal measured 400 m (1,312 ft). Beavers cut their own trees to strip it of branches and leaves. The largest tree cut down by a beaver was a cottonwood measuring (37 in) diameter and (110 ft) tall!

http://www.bbc.co.uk/nature/wildfacts/factfiles/615.shtml