To survive a nuclear blast, you’ll need to have your bunker at least three feet underground. However, it’s usually advisable to put it further underground. To survive a close strike, you’ll want to have the bunker at least 10 feet down and with five-foot thick reinforced concrete. However, knowing exactly how deep to build depends on the size of the blast you’re trying to avoid. Most bunkers don’t need to be more than 5 feet below the ground.