ThisIs A Circular Mill
Ants secrete a chemical as they venture out to look for food so that they know how to get back to the ant hill and so that they can follow each other without getting lost. It it happens that an ant walks into a circle while secreting this chemical, the ants will start moving into a circle. Eventually, the entire mound of ants will add to the circle until it becomes huge. They can last for days, either until all of the ants die of starvation or one of them happens to move out of place.