It’s May 2, 1972, and 75,000 feet above Hanoi – the most heavily defended airspace on Earth – three SR-71 Blackbirds scream through the stratosphere at Mach 2.5. The first aircraft drops altitude, its sonic boom rattles windows across the North Vietnamese capital like thunder. Exactly fifteen seconds later, the second Blackbird follows suit.Most people below think nothing of it – after years of bombing campaigns, they’re used to loud noises.But Deep inside Hoa Lo Prison – the notorious “Hanoi Hilton” – US Air Force Captain John Dramesi understands the message. Two sonic booms, fifteen seconds apart. Two dots in Morse code – the letter “I”. “Initiate.”