An Underground Mystery Had Engineers Stumped — Until It Rained

Deep beneath South Dakota, engineers at a former gold mine kept seeing something that should not have happened. During major rainstorms, a carefully controlled ventilation network suddenly weakened, shifted direction and sometimes pushed air the wrong way.

The disturbance threatened more than routine operations, because reliable airflow is essential wherever people work far below ground. Years of observations, new sensors and an unexpected clue eventually revealed how water falling through one shaft was reorganising the air across an enormous underground system.