Incredibly cool, if more than a little terrifying. I advise following the link to see the NPR video - we're talking megatons of man-made aurora borealis.
Christ, but the Cold War attitude towards radiation was fucked up. Let's not forget about all those glowing sheep in Nevada. You have to look back at the beginning of the Atomic Era and shudder; like giving a monkey a machine gun.
Happy Fourth! (h/t to Crooked Timber)