THE FIRST NUCLEAR reactor on earth went critical 2 billion years ago in western Africa when groundwater seeped into a rich vein of uranium ore. It controlled itself elegantly, its water moderator ...

The 80th anniversary of the atomic bombing of Hiroshima and Nagasaki is this week. It was the only time nuclear warheads were used during war. Here’s a look at the history and current U.S. stockpile.

Editor’s note: This article was sent to employees of the Los Alamos National Laboratory in 1995, at the 50th anniversaries of the bombings of Hiroshima and Nagasaki. I originally wrote the following ...