A The oceans cover more than 70 percent of the planet's surface, yet until quite recently, we knew less about their depths than about the surface of the Moon. The Moon has been far more accessible for study because astronomers have long been able to observe its surface, first with the naked eye and then with telescopes, both of which focus light. Until the twentieth century, however, no instruments were available for studying Earth's oceans: light, which can travel trillions of kilometers through the vast vacuum of space, cannot penetrate very far in seawater.
B It turns out that for penetrating water, the best instrument is sound. Curious investigators have long been fascinated by sound and how it travels in water. As early as 1490, the artist and scientist Leonardo da Vinci observed, "If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you." It was not until 1826 that two scientists, Colladon and Sturm, accurately measured the speed of sound in water. Using a long tube to listen underwater (as da Vinci had suggested), they recorded how fast the sound of a submerged bell traveled across Lake Geneva in Switzerland. What these investigators demonstrated was that water is an excellent medium for sound, transmitting it almost five times faster than its speed in air.
C Several factors influence how far sound travels underwater and how long it lasts, including particles, salinity, temperature, and pressure. Particles in seawater can reflect, scatter, and absorb certain frequencies of sound, just as certain wavelengths of light may be reflected, scattered, and absorbed by specific types of particles in the atmosphere. In 1943, Maurice Ewing and J.L. Worzel conducted an experiment to test the theory that low-frequency waves, which are less vulnerable than higher frequencies to scattering and absorption, should be able to travel great distances if the sound source is placed correctly. The researchers set off an underwater explosion and learned that it was detected easily by receivers 3,200 kilometers away. In analyzing the results of this test, they discovered a phenomenon known as the "deep sound channel." Sound introduced into this channel of water could travel thousands of kilometers with minimal loss of signal.
D The US Navy quickly recognized the usefulness of low-frequency sound and the deep sound channel. They developed the Sound Surveillance System (SOSUS), which involved underwater microphones, called hydrophones, placed on the ocean floor and connected by cables to onshore processing centers. Christopher Clark of Cornell University soon realized that SOSUS could be used to listen to whales. Using an SOSUS receiver in the West Indies, he could hear whales that were 1,770 kilometers away.
E Whales are the largest of Earth's creatures, yet they are also remarkably elusive. Scientists wishing to observe blue whales must wait in their ships for the whales to surface. A few whales have been tracked briefly in the wild in this way, but not over very great distances, and much about them remains unknown. However, by using SOSUS, scientists can track whales and position them on a map. Moreover, they can track not just one whale at a time but many creatures simultaneously. They can also learn to distinguish whale calls; researchers have detected changes in the calls of finback whales as the seasons change and have found that blue whales in different regions of the Pacific Ocean have different calls.
F SOSUS has also proved instrumental in obtaining information crucial to our understanding of climate. The system has enabled researchers to begin making ocean temperature measurements on a global scale—measurements that are key to understanding the workings of heat transfer between the ocean and the atmosphere. The ocean plays an enormous role in determining air temperature; the heat capacity in only the upper few meters of the ocean is thought to be equivalent to all of the heat in the entire atmosphere. For sound waves traveling horizontally in the ocean, speed is largely a function of temperature. Thus, the travel time of a sound wave between two points is a sensitive indicator of the average temperature along its path. Transmitting sound in numerous directions through the deep sound channel can provide scientists with measurements spanning vast areas of the globe. Thousands of sound paths in the ocean can be pieced together into a map of global ocean temperatures, and by repeating measurements along the same paths over time, scientists can track changes in temperature over months or years.
G Researchers are also using other acoustic techniques to monitor climate. Oceanographer Jeff Nystuen, for example, has explored the use of sound to measure rainfall over the ocean. Monitoring changing global rainfall patterns will contribute to understanding major climate changes as well as the weather phenomenon known as El Niño. Since 1985, Nystuen has used hydrophones to listen to rain over the ocean, acoustically measuring not only the rainfall rate but also the type of rainfall, ranging from drizzle to thunderstorms. By using the sound of rain underwater as a "natural" rain gauge, measurements of rainfall over the oceans will become available to climatologists. In this way, modern society continues to benefit from the investigations of those who, like Leonardo da Vinci, pursued answers to some of nature's fundamental questions.
