There was just one problem: the ants built their nest directly over a vertical ventilation pipe. When the metal covering on the pipe finally rusted away, it left a dangerous, open hole. Every year when the nest expands, thousands of worker ants fall down the pipe and cannot climb back out. The survivors have nevertheless carried on for years underground, building a nest from soil and maintaining it in typical wood ant fashion. Except, of course, that this situation is far from normal.
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The massive group tending the nest is entirely composed of non-reproductive female workers, supplemented every year by a new rain of unfortunate ants falling down the ventilation shaft.