Scientists classify fire ants as ground-nesting ants. The workers make tunnels into the soil – sometimes extending as far down as the water table. The tunnels expand outwards under the soil. There are many underground entrances so the foraging workers can go out to find food for the colony.
Inside the nest, the workers move the queen and the immature ants up and down in the tunnels. They move the queen to keep her temperature comfortable and constant.
In an area where fire ants have become established, there can be many separate colonies in an acre of land. Since the foragers are active in the early morning and around dusk, many homeowners do not see them. The grass and other plants often hide the foraging ants.
When the weather is rainy or very cold, fire ants often make hills or mounds. The workers bring the soil up from the tunnels. In rainy weather, the workers make mounds so they can move the queen up out of the damp ground. Many homeowners are surprised to see these mounds. These mounds are often the first sign that fire ants are active in their yard.
Fire ants often make their nest in sunny areas. Many times the ants make their nest beside a sidewalk or building foundation. Scientists suspect that they do this so they can take advantage of the sun's natural heating of the soil and the radiated heat from the concrete. In cold weather, the workers often make a mound. They move the queen up into the mound during the day when the mound is warmed by the sun. At night, they move the queen back into the tunnels.