A unique accumulation of workers (‘colony’) of the wood ant Formica polyctena Först., trapped within an old bunker for storing nuclear weapons, is described. The source of the ‘colony’ is a large colony nesting outdoors, on top of the bunker. Individuals that have fallen down through a ventilation pipe are not able to find their way back to the mother nest. In total darkness, they have constructed an earthen mound, which they have maintained all-year-round by moulding it and keeping the nest entrances open. Judging from the huge deposits of wood-ant corpses in the bunker, the ‘colony’ has survived for years. Through these years, the mortality has been more than compensated by new workers that fall down during the active season of the free-living colony outside, and at present the number of the bunker workers is counted in hundreds of thousands. The ‘colony’ has evidently produced no offspring, which is due to low (though relatively stable) temperatures and scanty food in the bunker.