Largest solitary animal burrow (living)

The largest burrows made by an extant species are the maternity dens of female polar bears (Ursus maritimus) used to shelter them and their cubs during the harsh Arctic winters. They are dug in snow banks, often on south-facing slopes near the coast. A den is approximately 3 metres (10 feet) long, over 2.5 m (8 ft) wide and 1 metre (3 feet) tall, and consists of up to three oval rooms and a narrow entrance tunnel. Insulated by the snow, body heat can raise the temperature of the den to as high as 4.4°C (40°F), even when it is far below freezing outside. If the roof becomes too thin or collapses, females may dig a temporary den or reoccupy an abandoned one.

In some areas, polar bears dig a summer den down through the tundra to the permafrost beneath, allowing otherwise overheated bears to doze on ice. While maternity dens are relatively temporary, some summer dens have been in use for hundreds of years.

Likely the first written account of a polar bear den dates from 15 April 1597, recorded in the diary of Gerrit De Veer (c. 1570– c. 1598), a Dutch officer who accompanied Willem Barentzs on his second and third voyages in 1595 and 1596 in search of the Northeast Passage: "...there came a great beare towards us, against whom we began to makedefence, but she perceaving that, made away from us, and we went to the placefrom whence she came to see her den, where we found a great hole made in theice, about a man's length in depth, the entry thereof being very narrow, andwithin wide; there we thrust in our pikes to feele if there was anything withinit, but perceaving it was emptie, one of our men crept into it, but not tofarre, for it was fearfull to behold."

The largest ever recorded animal burrows are palaeoburrows believed to have been created at least 8–10,000 years ago by now-extinct giant animals that can be found in their hundreds within South America but most commonly in southern Brazil. Some burrows are so huge that they were once thought to be caves, but no natural geological process is known that can create structures like these. One spectacular example that was discovered in southern Brazil by Heinrich Frank, a professor at Brazil's Federal University of Rio Grande, measures some 600 m (1,970 ft) long. It is approximately 1.5 m (5 ft) tall and varies from 1 to 1.3 m (3 ft 3 in-4 ft 3 in) in width. Confirming that they were created by animals is the presence of numerous claw marks on their inner wall surface, left behind by their creators. The precise vertebrate species responsible for these gigantic burrows is/are presently unknown. Giant ground sloths are the most popular identity suggested, but some researchers have speculated that gigantic armadillos or armadillo-like beasts called glyptodonts, also long extinct like the ground sloths, may have created at least some of them.