Under the

If you stop and think about it, the name "Water" would probably be a better name for our planet than "Earth." Dry land, or earth, only makes up about 30% of the planet's surface. Most of the rest is covered by oceans. Up until the mid-twentieth century, little was known about what the seafloors of our planet looked like.

There were several reasons for this lack of information. First and foremost, at certain points past the edge of the continents, the ocean gets really deep! Before the early 1900s, scientists really had no accurate way to measure the ocean's depth. The best method available was to take a line with a weight attached, drop it over the edge of a ship, and see how long it took the weight to reach the bottom. These "soundings," as they were called, worked fine in shallow water, but in the deep ocean, this method encountered real problems. Currents in the water would cause the line to drift in different directions, making it hard to tell when true bottom was reached.

When Alfred Wegener first proposed his continental drift theory, most scientists believed that the deep ocean bottom was simply a flat plain. There were a few clues that this might not be the case, however, such as ocean islands like Hawaii, the Galapagos, and Iceland, which seemed to rise up out of the seafloor. Then, in the late 1800s, the laying of the first undersea telegraph cable from Europe to North America indicated that there was some type of ridge running down the middle of the Atlantic Ocean. The real breakthrough are picked up by a receiver called a hydrophone. Since sound waves travel at a specific speed in seawater, by measuring the time it takes for a wave to travel out and return, a sonar operator can calculate in what direction and how far away an object is. Modern sonar systems are extremely sensitive. Not only can they locate submarines and sunken ships, but they are often used by commercial fishers to find fish. Geologists and oceanographers frequently use sonar to map changes in the seafloor, and a variation of the device is used by doctors in hospitals to take pictures of babies in the womb.

to understanding the true nature of the seafloor surface came during World War I with the development of two new devices: submarines and sonar.

Submarines were not a new idea. In fact, the first submarine was built back in the 1620s. However, these early vessels were tiny. They also leaked a great deal and were powered by the use of hand cranks and oars. Nor, before 1900, could submarines dive very deep or stay under water for more than a few hours at a time. Finally, in 1905, German engineers constructed the Unterseeboot 1, otherwise known as the "U-boat." This vessel, which was powered by strong diesel engines and electric motors, was the first truly modern submarine.

During the World War I, German U-boats terrorized Allied ships, sank many of them, and caused a great loss of life. While the Allies had submarines of their own, they were not as fast or as ma-neuverable as the U-boats. The Allies needed an edge against the U-boats, and it came with the invention of sonar.

The word sonar is derived from the words "Sound Navigation And Ranging." This device uses sound waves to locate underwater objects. First developed in 1916, sonar systems helped the Allies locate German submarines before they could attack ships. It turns out that sonar was also the perfect answer for scientists who wanted to learn more about what the ocean floor was like.

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