* Density is a measure of how much mass is packed into a given volume.

* Solid rock is the most dense of the options because its molecules are tightly packed together.

* Liquid water is less dense than solid rock. While water molecules are close together, they have more freedom of movement than in a solid.

* Ice (frozen water) is even less dense than liquid water. This is why ice floats!

* Gases like the atmosphere have the lowest density. Gas molecules are far apart and move freely.

Therefore, the solid rock beneath soils has the greatest density out of the choices given.