A burrow is a hole or tunnel excavated into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements and can be found in nearly every biome and among various biological interactions. Burrows can be constructed into a wide variety of substrates, and can range in complexity from a simple tube a few centimetres long to a complex network of interconnecting tunnels and chambers hundreds or thousands of metres in total length, such as a well-developed rabbit warren.
A wide variety of vertebrates construct or use burrows in many different types of substrate and can range widely in complexity. Mammals are perhaps most well known for burrowing, especially Insectivora like the voracious mole, and rodents like the prolific gopher, great gerbil and groundhog. The rabbit, a member of the family Lagomorpha, is a well-known burrower. There are estimates that a single groundhog burrow can occupy a full cubic metre, displacing 320 kilograms of dirt. Great gerbils live in family groups in extensive burrows, which can be seen on satellite images. Even the unoccupied burrows can remain visible in the landscape for years. The burrows are distributed regularly, although the occupied burrows appear to be clustered in space. Even Carnivora like the meerkat, and marsupials, are burrowers. The largest burrowing animal is probably the polar bear when it makes its maternity den in snow or earth.
Burrows by birds are usually made in soft soils; some penguins and other pelagic seabirds are noted for such burrows. The Magellanic penguin is an example, constructing burrows along coastal Patagonian regions of Chile and Argentina. Other burrowing birds are puffins, kingfishers, and bee-eaters.
Kangaroo mice construct burrows in fine sand.
The burrows produced by invertebrate animals can be filled actively or passively. Dwelling burrows which remain open during occupation by an organism are filled passively, by gravity rather than by the organism. Actively filled burrows, on the other hand, are filled with material by the burrowing organism itself.
The establishment of an invertebrate burrow often involves the soaking of surrounding sediment in mucus in order to prevent collapse and to seal off water flow.
Burrows are also commonly preserved in the fossil record as a type of trace fossil.