Twilight Tunnels

Water droplets can be heard falling from the tunnel ceiling and collecting in small puddles on the rocky ground. The air is cold and damp but not stuffy, even though the dark tunnels reach deep underground. The widely ramified tunnel system, consisting of horizontal and vertical tunnels, now and then interrupted by crevices and ravines, was dug into the rock over many years by a slate dragon. No one knows who first laid siege to this place, the dragon or the slate rock. And then there is the impenetrable darkness that even magic cannot conquer. Only daylight can dispel the shadows. But how is it supposed to get hundreds of meters underground?