Why is being underground safe during a tornado?

If you are underground the ground will protect you from object that would come from the side and the ground or building overhead will shield you from above. Since the bottom floor of a building is usually the last thing to fail in tornado, it will shield you from all but the most violent tornadoes if you are in the basement. You are even safer in a storm cellar as you have the ground itself and a layer of concrete above you. Additionally, the lower you go the slower the winds are.