Though these passages reveal to us belief during the Crusading age in the presence of heat and fire in the inside of the earth, the teachings of Plato and many of the Church Fathers that the sources of volcanic fires spring from immense subterranean reservoirs of fire do not appear to have been given much credence. Though the earth’s crust and even its innermost heart might be interpenetrated with cavities into which air, water, and fire enter, confidence in the essential solidity and massiveness of the earth prevailed, and theories which would admit of the presence of bodies of water or of fire of any great extent within the heaviest and most solid of the elements were not regarded as worthy of serious consideration. Hell, however, was almost universally placed at the very center of the earth by medieval theologians and geographers alike.[1035] EARTHQUAKES The majority of medieval writers believed that earthquakes are caused by the same physical forces as those which produce volcanic eruptions, the violent stirring of winds,[1036] vapors, or exhalations within the earth’s crust.[1037] In the _Quaestiones naturales_ of Adelard of Bath, which takes the form of a dialogue between uncle and nephew, the nephew finds it difficult to reconcile the stability and immobility of the earth, which his uncle had just demonstrated, with the well-known fact that the earth sometimes quakes and trembles. To this Adelard replies that, while it is true that the earth may occasionally move in particular localities, it does not move as a whole[1038] and that earthquakes are caused by the air contained within the earth and have nothing whatever to do with the intrinsically stable qualities of the earth as a globe. He then proceeds to give the Aristotelian explanation of the causes of earthquakes.[1039] The _De imagine mundi_, followed by the _Image du monde_ and by Bartholomew Anglicus,[1040] also assigns the same causes; and William of Conches explains[1041] that earthquakes are the result of waters descending into underground hollows where they encounter vapors, condensed into cloudlike form by the coldness of the earth’s interior; these vapors, in turn, produce telluric movements by forcing their way to the surface. Neckam repeats much the same explanation but adds the usual allegorical lesson:[1042] the land symbolizes the Church, which, although on the whole serene and firm, may well be shaken now and then by purely local troubles and disturbances.