巴士海峽
After transiting the Bashi Channel the Echo established a second patrol area in the Philippine Sea south of Okinawa. This area of the ocean had some of the worst possible acoustical properties. It was often crossed by merchant traffic and at night the biological noise and frequent rain showers were deafening to sonar. Maintaining contact became even harder than before, making it necessary for "Guardfish" to trail at closer and closer ranges.