Possible Clues:

• The first one was dropped on Hiroshima
• A grave treacherous mob is hugely destructive
• Nuclear weapon
• Manhattan Project project
• The first one fell on Hiroshima
• A grave old doctor spells out a threat to humankind
• Weapon of mass destruction
• Explosive device discovered in a vault gets mob to scatter
• It first fell on Hiroshima
• A temperature, to tw o old doctors, can be a big killer