I have a vivid childhood memory of visiting my grandmother's house near Oak Ridge, Tennessee.

Above the bed where I slept, hung a photo of my grandfather standing in front of a

mushroom cloud. I'd stare at his image on my grandmother's wall for hours. I didn't know what it was. I didn't know what it

meant. I never met my grandfather. He died before I was born.

But now I know what he did. Now I know what he helped to create.

Last night's target for the first atomic bomb

was the city of Hiroshima on the shores of the

inland sea west of Kobe. My grandfather worked on the bomb that dropped on the

city of Hiroshima 75 years ago. As I've grown older, I've tried to make sense of my grandfather's

choices. I've tried to make sense of what he was involved with. And I'm still

trying to make sense of what humanity unleashed when it dropped that bomb. The only description that I can come up with reminds me of years gone by when they used to pave the streets with tar and that's exactly what the city of Hiroshima looked like.

To understand, I've gone back to the beginning,

back before Hiroshima and Nagasaki,

before my grandfather got involved at Oak Ridge,

and it led me to a fascinating guy,

all but forgotten by history.

Leo Sillard.

Sillard was just a hugely curious person.

Leo Seelard.

Seelard conceives the idea of a nuclear chain reaction.

He's way ahead of a lot of people who didn't foresee the threat to the world community,

the future of civilization that was unleashed.

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