Leonardo’s vision of the planet earth seems to be fairly straightforward. The “body” of the earth is contained within a sphere of water. This protrudes from the water in places because it is irregular and appears as landmasses and mountains, as shown in the upper diagrams on the right of this sheet. The spherical shape of the earth is maintained because all of the elements adopt a spherical shape around their own centre of gravity. However, things are not quite as simple as that!

The earth was not a solid mass. In the fifth drawing from the top, Leonardo illustrates the layered nature of the earth. It contains vast subterranean caverns formed by the erosive powers of water that flowed through the body of the earth.

These caverns continually collapse and fall in upon themselves. As they do, the heavy matter is pulled towards the centre of the earth and the light pushed towards the periphery, where it forms new mountains and landmasses. And so in the dynamic formation and reformation of the elements of the earth a perfect state of equilibrium is maintained.

In Leonardo's words
I consider that the highest mountain there is on the earth is as far above the surface of the sphere of water as the greatest depth of the sea is below the surface of the sea. It follows therefore that if one were to fill up the part wanting in the sea with the excess of earth, the earth would remain entirely spherical and covered by the sphere of water.