Every living organism relies on water to survive, and the distribution and movement of water (known as the water or hydrologic cycle) is taught from primary school through to university.
This pattern of water movement is well understood. Water evaporates, condenses in the clouds, and then returns to the earth as precipitation (rain or snow). Some soaks into the ground (infiltration) and is stored as soil water and groundwater. From there, it is transpired back into the atmosphere by plants. Some becomes stream flow, eventually making its way back into lakes or the ocean. An obvious enough process, but did we always understand it?
The ancient Greeks had quite different ideas about the water cycle. Thales of Miletus (late 7th–early 6th Century BC) believed streams and rivers emerged from a vast subterranean freshwater lake, connected to the surface of the world by chasms. Water then flowed out via surface springs into the rivers of the world.
In the 4th century BC, Aristotle described the water cycle more accurately, but like Thales, remained convinced that subterranean water was the main source of stream flow. He wrote that it was absurd “if one were to suppose that rivers drew all their water from the sources we see (for most rivers do flow from springs).”
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