It's been 70 years since the US became the first nation to drop an atomic bomb. So far, it's the only one. But how long will that last? Today there are some 16,000 nuclear weapons in nine countries, and they're many times more powerful than those used against Japan. Despite arms reduction agreements, the US and Russia still have massive arsenals, and now both countries are “modernizing” their stockpiles — ostensibly for defensive purposes. But each side suspects the other of thinking offense -- as memories the more than 100,000 people who died in Hiroshima and Nagasaki fade into the past.