Depends. If it just becomes mushy or freezes very little, probably no damage will be done. if it freezes solid, you will have severe internal engine damage. Cracked block, or cracked head is common.
It explodes and the apocalypse happens.... What do you think happens? it freezes
the water freezes and expands.
As the water freezes, it will expand.
you get frostbite
It burst out.
It can break the water pump housing, engine block, radiator. Major damage.
Its absorbed think what happens when it freezes