At 17,000 feet beneath the surface, the temperature of ocean water is just above freezing, oxygen is sparse and currents are relatively calm. In other words, ideal conditions for preserving an airplane that might have crashed into the depths nearly 70 years ago, according to marine explorer David Jourdan, who hopes to answer one of aviation's greatest mysteries: the fate of famed pilot Amelia Earhart. Jourdan and his Maine-based company, Nauticos, plan to launch an expedition in the spring using sonar to sweep a 1,000-square-mile swath of ocean bottom west of tiny Howland Island in the Pacific Ocean. It...