Why do tornadoes have a big hole in the middle?

The hole in the center of a tornado is formed in a similar way to the eye of a hurricane. Low pressure inside the tornado pulls air in. As the air moves in it starts spinning faster. At a certain point the tornado is spinning so far that air cannot make it all the way to the center. Instead, air moves down the center. This process is called vortex breakdown and usually only occurs in intense tornadoes.