For the alchemists of the Renaissance, it was a well established fact that mercury was to the metals as blood was to the body. Though the untrained knife would often bend or break while attempting to find untapped veins, a skilled practitioner could find a pulse within any ore, and draw forth a fountain of quicksilver with a single, well-placed incision. Every metal could be made to bleed this same lustrous ooze, from profane lead to sacred gold.
Knowledge of this art was not native to humanity, however; it was passed down from the giants, who taught their craft to mankind during the short period of history when both walked the earth. For such a purpose, these beings constructed devices as long as a cavalryman's spear, though they looked like little more than sewing needles in their colossal fingers. Their size betrayed no lack of precision, for the objects came to a point as thin as the tip of a syringe.
On the tail ends of these "mountain drinkers," as the devices came to be known, were small, valve-controlled spigots. These mechanisms provided a trajectory for quicksilver to flow outward from the mountain once a vein was struck, similar to spiles used by forest dwellers to extract sap from trees. In particularly metal-rich mountains, the resultant flow was sometimes greater than those of waterfalls flowing directly alongside.
Once this vital fluid was drained from the mountain, the metals within were depleted of all vitality and maturity. No matter what had once filled the mount’s bowels, once the tapping of quicksilver was complete, only stone and lead remained. The mercury that escaped was not kind to its surroundings, either, and left behind a path of lifelessness through drowning and poisoning alike. For this reason, the mountain drinkers found brief use as weapons of mass destruction before being abandoned and lost to history altogether.
It's not clear what use the giants had for drawing forth entire mountains worth of mercury at once. The most commonly held belief among anthropologists is that the lagoons of mercury which formed in the immediate aftermath were simply intended to be used as mirrors. These pools were large enough for the giants to view themselves in their entirety, a luxury which creatures of a smaller stature often take for granted.