Here, in this gallery, LIFE.com presents rare and (mostly) unpublished pictures made in the Nevada desert by photographer Loomis Dean shortly after a 1955 atomic bomb test. These are not “political” pictures. They are eerily beautiful, unsettling photographs made at the height of the Cold War, when the destructive power of the detonation was jaw-droppingly huge — but positively miniscule compared to today’s truly terrifying thermonuclear weapons.