Twenty-five years ago, two thieves committed the greatest art heist in American history. Dressed as police officers, the pair entered the Isabella Stewart Gardner Museum in Boston at 1:24 a.m. via a side door by ringing the buzzer, persuaded the only security guard sitting at the main desk to step away from it (and from the sole button that could trigger an alarm), and then restrained him and the only other guard on duty before making off with 13 artworks worth an estimated $500 million.