The term "ant colony" describes not only the physical structure in which ants live, but also the social rules by which ants organize themselves and the work they do. Ant hills formed by soil nesting ants are familiar sights. However, these mounds are not actually ant colonies. Instead, they mark the entrance and exit to the colony. These mounds are made up of the dirt, sand and other material the ants must remove as they dig the underground tunnels and chambers in which they nest. In fact, most ant colonies stretch deep underground, some even as deep as 25 feet.

Source: Terminix