Deep Hollows
The Deep Hollows are the primary (and only consistent) source of [Avaldrite], and are thus vital to oxygen production. Located at least a dozen miles lower than the majority of the civilised world, they consist of pockets of vacuum that open up apparently spontaneously in untouched rock. The discovery of a new hollow is almost garaunteed to cause massive loss of life, injury and damage to property, due to the tremendous forces of suction they exert when freshly exposed to a pressurised environment. It is also however the cause for great celebration, as it signifies an extension of human existence beneath the surface.
Deep Hollows have been measured externally to be at most a mile in diameter, though reports from those that venture inside for the purpose of mining avaldrite or exploration have claimed that in some, one may walk the sides for days without encountering a bend, curve, or one's starting point. It is because of this, along with occasional inexplicable winds and other curious atmospheric phenomena, that many scholars believe the caves to be of variable dimensions, or else not bounded by conventional laws of space and depth.
Even with glowing avaldrite deposits, the Deep Hollows are curiously lightless places, anything over a hundred feet away appearing practically nonexistent. Aside from the precious mineral, huge deposits of finely powdered rock can be found scattered on the rock floor, which is occasionally flooded by brief, silent deluges of between an inch and forty feet of water. No point of origin or dispersal for these deluges has ever been discovered. Movement inside the hollows is held to be extremely risky, as when not within the sight of others, individuals and even entire companies have been known to vanish. Despite this, the vital mining operations continue, regardless of the danger. No remains are ever found; this may be related to the curious disappearance of human and animal remains within the hollows if left unwatched. No species is known to populate the hollows other than the blind and unremarkable [Sootmice], who remain the sole settlers of these barren lands. Vague sightings of massive oblong stone formations have resulted in some academics connecting these structures with the elusive [Granters of Avaldrite], though there is no evidence to suggest this.
Arthur Patrimoch, Head Archivist of Slatearch