Fire ants live in an extensive network of tunnels in the soil. How deep this network extends into the ground depends on age and size of the colony, soil texture, and depth of the water table.
Tunnels in fire ant nests have been found to a depth of 10 feet or more, but most tunnels are shallower, starting just beneath the soil surface. Colonies in clay soils have deeper tunnels than those in sandy soils. Mounds are not necessary for colony survival as long as there is a dark, moist area for protection of the queen.
A rule of thumb to use when drenching mounds with an insecticide is to assume that there will be tunnels an inch and a half below ground for each inch in height above ground. For flat mounds, assume there will be tunnels an inch and a half below ground for every inch in mound diameter. This is why it is necessary to use more liquid insecticide on a bigger mound than on a smaller mound.
Find more information about fire ants in eXtension's Imported Fire Ant Resource Area.