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SHIPPING is making the Pacific Ocean 10 times noisier than it was 30 years ago—and this could unsettle marine mammals. Rex Andrew and his team at the University of Washington in Seattle looked at recordings from US Navy hydrophones anchored 40 kilometres off the coast of California. They found that both low-frequency (around 20 to 80 hertz) and mid-frequency (between 200 and 300 hertz) sound was around 10 decibels higher—equivalent to 10 times louder—than it was in the 1960s (Acoustic Research Letters Online, vol 3, p 65). The scientists think that ships’ propellers are the main cause and alternative means…