As the limestone in which the ores are imbedded does not form a solid compact mass, but is of a loose nature, most of the galleries had, from the very beginning of the mining operations, to be artificially propped. Until the end of the last century wooden beams were chiefly used for this purpose—a construction which frequently gave rise to fearful fires. Thus on March 16, 1803, the workmen saw a thick smoke issuing from several of the deepest galleries. It rose higher and higher, and spread through the upper galleries of the mine. No fire was to be seen, no sound of flames was to be heard, yet there could be no doubt that the mine was burning. Some of the workmen boldly undertook to descend to the scene of the fire, but in vain; they were obliged to flee before their enemy, for the smoke was not only dense and suffocating, but so impregnated with poisonous vapours that no living being could exist within its reach. An attempt was now made to smother the fire by shutting out the air. All the galleries were blocked as near as possible to the supposed seat of the fire, as well as the two shafts which led to the upper world. The mine remained thus closed during five weeks, but to no purpose; for when it was reopened, the fire burst forth more furiously than before. The flames now howled fearfully from the bottom of the pit, and the mercurial and sulphurous fumes which rose from it threatened instant death to anyone who should be rash enough to approach them. The director of the mine now resolved to flood the works as a last resource against the ravages of fire. A river was turned into the perpendicular shaft, and ran during two days and three nights into the pit. On the first day there was no perceptible effect, but on the second—whether it were that the vapours produced by the meeting of the antagonistic elements were striving for an outlet, or that new inflammable gases had been formed—a most terrific explosion took place, which made the whole mountain tremble as if shaken by an earthquake. The huts close to the opening of the shaft were rent to pieces, the stone houses at the foot or on the slopes of the hill fell in with a tremendous crash, and the inhabitants of Idria, fancying the day of judgment had arrived, fled in terror to the hills.