Reports of the low-frequency buzz began in September and theories range from wind to lamps to subsidence

As twilight settled over Omagh this week, factories gradually fell silent, the rumble of traffic ebbed and a half moon glowed in an inky sky, setting the stage for a nightly mystery: the hum.

Those who hear it describe a continuous, low-frequency drone, or buzz, and say that once heard it cannot be unheard, merely endured.