On April 15, at four o'clock in the morning, a small group of scientists found their way to a secret location. A light wintry mix of rain and snow was falling. The lousy weather was a relief because it meant even less of a chance that someone might randomly pass by.Today on the show, NPR science correspondent Nell Greenfieldboyce unearths why a new generation of scientists is digging up seeds under the cover of night buried 142 years ago.