Many of the more powerful weapons cause the shape of the earth to change, for example by blasting craters, forming volcanos, or digging holes. These effects all happen in real time by deforming the mesh of the earth model. The built in Unity physics system was too slow to handle these changes and deal with collisions in real time, so a custom collision system was created which allows missiles to impact the side of volcanos or fall into craters
The earth after a battle. Large craters and volcanos are visible.
The earth mesh showing deformation caused by weapon effects.
And finally, a video showing the deformation effects in-game: