Figure 14.20: Dolphins and whales use echolocation, a natural sonar system, to navigate the
ocean. ( 15 )
for every 10 meters of depth, the pressure increases by nearly 1 atmosphere! Imagine the
pressure at 10,000 meters; that would be 1,000 kilograms per square centimeter (14,700 lbs/
sq in). Today’s submarines usually dive to only about 500 meters; to go deeper than this
they must be specially designed for greater depth (Figure14.21).
In the 19th century, explorers mapped ocean floors by painstakingly dropping a line over
the side of a ship to measure ocean depths, one tiny spot at a time. SONAR, which stands
forSoundNavigationAndRanging, has enabled modern researchers to map the ocean floor
much more quickly and easily. Researchers send a pulse of sound down to the ocean floor
and calculate the depth based on how long it takes the sound to return. Of course, some
scientific research requires actually traveling to the bottom of the ocean to collect samples
or directly observe the ocean floor, but this is more expensive and can be dangerous.
Inthelate1950s, thebathyscaphe(deepboat)Triestewasthefirstmannedvehicletoventure
to the deepest parts of the ocean, a region of the Marianas Trench named the Challenger
Deep. It was built to withstand 1.2 metric tons per square centimeter and plunged to a
depth of 10,900 meters. No vehicle has carried humans again to that depth, though robotic
submarines have returned to collect sediment samples from the Challenger Deep. Alvinis a
submersible used by the United States for a great number of studies; it can dive up to 4,500
meters beneath the ocean surface (Figure14.22).
In order to avoid the expense, dangers and limitations of human missions under the sea,
remotely operated vehicles or ROV’s, allow scientists to study the ocean’s depths by sending
vehicles carrying cameras and special measuring devices. Scientists control them electroni-
cally with sophisticated operating systems (Figure14.23).
Features of the Seafloor
Before scientists invented sonar, many people believed the ocean floor was a completely flat
surface. Now we know that the seafloor is far from flat. In fact, the tallest mountains and