According to the modern theories, most of the craters that cover the Moon's surface were formed as a result of collisions with large meteorites and small asteroids. Plenty of those were falling on the lunar surface approximately 3.5 billion years ago.
This model demonstrates the formation of craters as meteorites collide with the lunar surface. The main window schematically represents the upper layers of the Moon several billion years ago, when the intensive meteorite showers were taking place.
Press "Run" button. If a small meteorite collides with the lunar surface, it breaks the lithosphere and creates a large crater. The debris from the explosion covers the meteorite from the top, and forms a characteristic hill in the center of the crater. "Stop" button brings the animation to a halt.
Press "Reset" to return the model to its initial state, and select a large meteorite for the simulation. As the meteorite breaks through the lithosphere, the upper layers of its liquid mantle splash on the lunar surface. The liquid mantle gradually solidifies, and forms the dark lava fields. The largest of these lava fields are known as lunar seas.