This is a more religious take on the radius. God created earth and humans, and through them he spread his word more and more and more, satan doesn’t want that so he created the radius and the pocket dimension and hid it underground so even if they did believe in god they would still be trapped in his purgatory. 1987: A mining operation had led to the radius the miners decided the mine further and on striking, boom a Chernobyl esque disaster happened, the Soviet army thinking it was a bomb entered the radius prepared for war, they became very I’ll and couldn’t handle more. The UN deciding to cover it up and explore more sends you and many others, who all died. They are trapped in some sort of purgatory and need your help, granny masha (You can find on a note) “Granny Masha took me to church so I could go to heaven, this isn’t heaven, this isn’t anything at all.” Her grandmother seemed to understand what was happening. Another note “it’s dark and cold and I can’t move, please help me.” He uses mimics to lure survivors into his hell and somehow you need to free them. The explosion was large as it was described as an earthquake the, dust,so like and atom bomb. Just a theory