Depth underground is important for nuclear shelters as the covering of earth is the main means of mitigating radiation levels. Standard depth for new bunkers is just over 2.5 metres (8.5 feet), designed to reduce an overhead burst of deadly radiation from a one-megaton bomb to less than the effect of a single chest X-ray (0.01 sieverts) – which is harmless.
The sievert is used to measure the biological effects of radiation. From the gamma and neutron sources that characterise the deadliest parts of a nuclear blast’s radiation burst, a person in good health can be exposed to a dose of up to 0.25 sieverts and suffer no ill-effects.
Curiously, a smaller 500-kiloton nuclear bomb exploding in the air can be more lethal than a much larger weapon on the ground. This is because nuclear air bursts produce much more deadly neutron radiation than surface bursts.