Squirrel burrows have an average opening of 4 inches in diameter and can be interconnected through underground tunnel systems with multiple entrance and exit holes. While they can dig quite deep, the average burrow is usually less than three feet deep. Ground squirrels like to burrow underneath rocks, logs, or other structures that give them some protection from digging predators. Their burrows will often be on sloped ground where flooding is less likely and visibility is good.
To learn more, read this paper on California ground squirrel burrow structure . It also has information on control implications.
Burrows are abandoned by squirrels for a number of reasons:
- squirrels have died
- food sources have decreased in surrounding areas
- environmental habitat has changed making the area undesireable to squirrels (i.e. high moisture, invading high grasses/shrubbery in burrow area—remember, squirrels prefer short herbage for better visibility)
- high invasion of pest species (fleas, ticks, biting insects, etc.) or presense of a zoonotic disease
Signs of an active burrow:
- food debris and litter (shells, greens clippings, seeds, etc.)
- tracks
- fresh deposits of dirt or soild from burrow
Signs of an inactive burrow:
- spider webs
- leaves and debris in entrance
Some other animals will occupy abandoned squirrel burrows. It is important to distinguish which burrows are inhabited by ground squirrels and which are used by these other species. Your local county agriculture commissioner office is often a good source of information about identifying ground squirrel burrows and burrows occupied by other species.
Some general guidelines for distinguishing other burrows from squirrel burrows:
Kangaroo rat burrow identification 
San Joaquin kit fox burrow identification
Burrowing owl burrow identification