The oceans of Earth cover more than 70 percent of the planet's surface, yet, until quite recently, we knew less about their depths than we did about the surface of the Moon . Distant as it is, the Moon has been far more accessible to study because astronomers long have been able to look at its surface, first with the naked eye and then with the telescope both instruments that focus light. And, with telescopes tuned to different wavelengths of light , modern astronomers can not only analyze Earth's atmosphere but also determine the temperature and composition of the Sun or of stars many hundreds of light-years away. It turns out that, for penetrating water, the phenomenon of choice is sound. Water is an excellent medium for sound transmission.
Sound travels almost five times faster in water than in air.
Had the world known how to harness the extraordinary ability of sound to travel through water, in 1912, the Titanic might have had some warning of the iceberg that sent the luxury liner to the bottom of the North Atlantic and took the lives of 1,522 passengers and crew. Using these tools scientists and engineers went on to devise ever more sophisticated instruments for finding submarines during both World Wars. Today, researchers apply their knowledge of how sound travels underwater to carry out myriad tasks, such as detecting nuclear explosions, earthquakes, and underwater volcanic eruptions. And just as astronomers use light to probe the secrets of the atmosphere, scientists in a field called acoustical oceanography use sound to study the temperature and structure of Earth's oceans measurements crucial to our ability to understand global climate change. Researchers in biological acoustics also use sound to study the behavior of marine mammals and their responses to human-generated underwater noise, helping to guide policies for protecting ocean wildlife.