|Every 3.5 Inches of earth reduces the amount of gamma rays by
1/2.So you can see that five zones of earth, 3.5 inches thick will reduce deadly gamma
rays to a survivable level.
Remember every single gamma ray that you
get reduces your life by one day.
I recommend a minimum of two feet of earth over any shelter. My days are
precious to me. How about you?
Relize that if you get 30 rads of radiation you have shorten your normal life
span by a month.