How do you make a very good underground bomb shelter?

Keep in mind that a good underground bomb shelter is supposed to protect you even after a bomb explodes. It should protect you from nuclear fission and radiation, whose effects can still be felt even after several years. Your shelter should also be accessible from different parts of your place. People will be staying in the shelter for some time so make sure to provide good ventilation, ample storage areas, and comfortable living conditions. You can go as far as including an air filtering system that removes radiation from the air.