Two years later, the Blackbird was retired. In June 1991, the first in a series of unexplained shock waves rolled across the Los Angeles basin, rattling doors and windows and making people think of earthquakes. But they were not earthquakes, and the military adamantly denied that any of its vehicles caused the booms. In May of this year, I consulted with Dom Maglieri, an ex-NASA sonic-boom expert who has played a key role in the development of low-sonic-boom aircraft. We studied 15-year-old seismograph data from the California Institute of Technology, whose uniquely sensitive sensors could actually track the booms. "The data showed something at 90,000 feet, Mach 4 to Mach 5," Maglieri says now. The booms did not look like refracted, "over the top" booms, as other reports had indicated-booms from aircraft miles away that had traveled up through the atmosphere and bent down toward Los Angeles. The booms looked like direct overflights by a supersonic airplane that no one admitted to owning. "The signatures are awfully different," Maglieri says.