For Leonardo, even the Earth is a vast organism whose motions are governed by the universal mechanical laws of Nature. He subjected the Earth's "members" to the same refined anatomy that he practiced on machines and the living body. Leonardo thus discovered in the planet, as in man, a ceaseless internal circulation of waters ascending via distillation processes from the ocean depths to mountain-tops, from which they flow back down to the sea.
Water, whose motion fascinated Leonardo, is the basic dynamic element of his geological universe: it excavates the Earth's crust, lays mountains bare through erosion, and provokes volcanic phenomena and earthquakes. Leonardo continually refers to the body-Earth analogy, endowing the planet with a vegetative life: "Its flesh is the soil; its bones are the rocks which support the Earth; its blood is underground water streams."
Leonardo explains the Earth's structure in terms of strictly mechanical principles and laws, whether he is investigating its center of gravity, tides, the origin of mountains and geological strata, wind erosion, or the destruction caused by floods and earthquakes.