The water cycle is such a basic concept that is taught in primary school science, that it is almost impossible to imagine what it would be like to not know about it. I recently stumbled upon a brilliant 1970 book - The History of Hydrology by Asit K. Biswas - which showed me that it was far from obvious to the philosophers and scientists of the 17th century.
The biblical text Ecclesiastes mentions in passing that rivers flow back to their source, and this idea held sway over philosophers for centuries. It led to multiple speculative hypotheses of how the water from the ocean could reach the source of the rivers and feed their ceaseless flow. Of these possible mechanisms, there is perhaps none more fascinating than that imagined by the German Jesuit professor Athanasius Kircher.
In 1664, Kircher wrote a book called Mundus Subterraneus (The Subterranean World) which became a standard geology textbook in the late 17th century. In that book, he elaborated on his theory of how the waters of the rivers were replenished.
Kircher thought that there must be great caverns filled with water (he called them hydrophylacia) inside the main mountain ranges of the world. According to him, God created these water-filled caverns in the mountains so that the rivers flowing from them could be used for navigation and irrigation by man.
It seemed logical to Kircher that since the quantity of water that could be present in the mountain caverns was not infinite, there must be some mechanism for water from the oceans to reach these caverns.
Kircher visualised seawater passing through openings in the sea floor to the hydrophylacia via underground channels. Given below is an illustration of the same from Kircher's Mundus Subterraneus. In the diagram, the underground channels are shown in a dark shade while the rivers are shown in a lighter shade. The whirlpools indicate places where Kircher believed the sea floor opened into the underground channels.
Kircher knew that he still needed to explain how water travelled from the sea bed (at a lower level) to the mountain caverns (at a higher level). After considering various mechanical processes of raising water through a pipe, Kircher identified two possible ways.
The first was by the action of wind and tides on the ocean surface. He believed that these factors could produce the pressure necessary to push seawater uphill through the underground channels to the hydrophylacia.
Another possibility that he saw was that water could be raised to a higher level by the action of fire. He imagined that there existed several underground caverns on the way with fire inside. Such fiery caverns vapourised the water, which went uphill through the channels and condensed into water in the mountains. It is not clear to me if Kircher thought that this could also explain why river water was not salty, since that is not discussed in the section on Kircher in Biswas' book.
An added advantage of the fiery caverns hypothesis was that it could explain the existence of hot springs. Kircher explained that wherever the fiery caverns were close to the outlet of the underground channels, the water came out hot. If the water had to travel for a long distance after being heated, it came out cold.
It is clear that Kircher was deeply interested in underground springs and water bodies. He had identified a critical problem in hydrology, but his explanations and ideas left much to be desired. It would be a few decades before Edmond Halley - he of the Halley's comet fame - carried out calculations that showed that the evaporation from the Mediterranean Sea was more than enough to feed all the major rivers in Europe, effectively placing evaporation from the seas as the central factor driving the water cycle.