To understand the mystery of the Underbath, what lies beneath the surface, requires a flight of imagination towards facts that are stranger than fiction and a science predicated on imagination and observation as Geology as a science did not exist before the latter half of the eighteenth century.
Prior to this there were many theories that combined curiosity and close observation with theology; vivid imagination and superstition with folklore and local knowledge.
As is shown in these questions from the father of modern chemistry Robert Boyle, natural philosopher, physicist and pioneer of the scientific method, who in 1666 asked:
Whether mists used to rise from Grounds stored with minerals? Whether the place be more than ordinarily subject to Thunder and Lightening… as likewise to Nocturnal Lights and fiery Meteors?
Rock formation and ore generation were the subject of hot debate and many conflicting schools of thought.
If minerals grew in veins like vegetables, could they therefore be harvested and regenerated?
During 15th & 16th C. there was a widely held belief that mineral formations were influenced by rays from the stars and later, more specifically, from the sun moon and five known planets.
The idea growing out of the coincidence that the seven heavenly bodies correlated to the (then) seven known metals and the fact that before Copernicus the earth had been considered the centre of the universe so that it logically followed that all celestial rays were concentrated upon the Earth and stored in a vast fiery hollow at its core. Vapours from this core, acted upon by rays from one or other of the seven heavenly bodies imparting its own specific metallic characteristic – lead, iron, tin, copper etc.
Other theories included, in A Philosophical Essay Declaring the Probable Causes Whence Stones Are Produced in the Greater World from Which Occasion Is Taken to Search Into the Origin of All Bodies, Discovering Them to Proceed from Water and Seeds (1672), Sir Thomas Sherley’s idea that metallic seeds were implanted by rising vapours in bedrock adjacent to fissures and that water acting on these seeds produced a mythical mineral juice called Ghurr or Thurr from which metalline earths gradually grew.
Whilst Georg Bauer, whose pen name was Latinised as Georgius Agricola in De re metallica (On the Nature of Metals (Minerals)) rejected the planetary and the seed theory and, anticipating modern theory by some three centuries, argued that the fissures in which mineral deposits occurred were formed at a period subsequent to the formation of the mountains and were likely to have been caused by erosion with water, either from within the earth or rain soaking into the rocks taking up mineral matter to become a lapidifying juice. The juice then flowed into the pre-existing opening or fissures forming mineral veins.
Water, heat, solution, absorption, flow, time… no matter how seemingly outwardly bizarre each theory might be, it would add something to the sum of knowledge and, together with the accumulation of stratigraphic knowledge, the understanding of the geological layering, often based on the shrewd observations and local knowledge of miners, it would in time reveal the story of the geology of the Underbath.