In the sketches on the right of this sheet Leonardo illustrates his global theory.

He believed the centre of the earth to be a cavern filled with water, which constantly rises to the surface through a system of subterranean veins. These subterranean waters erode the earth’s interior, causing the enormous internal caverns to collapse. As these caverns crumble, water fills them and the relative centres of gravity of the earth and the water constantly change.

The surface of the “sphere” of the liquid sea remains at a constant level, but the earth protrudes to different heights and different places, and is radically transformed over time. From this idea, Leonardo developed his hypothesis on the origin of land masses and mountains.

In Leonardo's words
The great elevations of the peaks of mountains above the sphere of the water may have resulted from the fact that a very large portion of the earth which was filled with water, that is to say, the vast cavern inside the earth, may have fallen in a vast part of its vault towards the centre of the earth, being pierced by means of the course of the springs which continually wear away the place where they pass…It is of necessity that there should be more water than land, and the visible portion of the sea does not show this; so that there must be a great deal of water inside the earth, besides that which rises into the lower air and which flows through the rivers and springs.