An ant mill is a bizarre phenomenon where army ants separated from their main foraging party lose the pheromone trail and begin following each other in a continuous rotating circle. This behavior, often called a "death spiral," can persist until the ants die of exhaustion from endlessly circling.

First documented in 1921 by William Beebe, who observed a massive 370-meter circumference mill, the behavior takes each ant about two and a half hours to complete one revolution. The phenomenon has been reproduced in laboratories and through ant colony simulations, demonstrating how self-organizing structures in ant colonies can produce unexpected outcomes.

This behavior occurs when a slight deviation in the ants' path triggers a feedback loop, causing them to follow one another rather than the original pheromone trail. Similar circular behaviors have been observed in processionary caterpillars and fish, highlighting how collective animal behavior can sometimes lead to counterproductive results.