The sea floor then drops off steeply along the continental slope, the true edge of the continent. The smooth, flat regions that make up 40% of the ocean floor are the abyssal plain. Running through all the world’s oceans is a continuous mountain range, called the mid-ocean ridge(“submarine ridge” in Figure 14.23).

Marine geology has strong ties to geophysics and to physical oceanography. Marine geological studies were of extreme importance in providing the critical evidence for sea floor spreading and plate tectonics in the years following World War II.

Marine geologists rely on sonar (short for “sound navigation and ranging”), which is the use of underwater sound waves. Sound travels at a constant velocity (speed) in water, so the time it takes for the sound wave to travel through the water and echo back to the ship illustrates variations in the seafloor.

The seabed (also known as the seafloor, sea floor, or ocean floor) is the bottom of the ocean, no matter how deep. All floors of the ocean are known as ‘seabeds’.

174.5 feet

The simple answer is that not all of the ocean floor is made of sand. The ocean floor consists of many materials, and it varies by location and depth. In the deepest parts of the ocean, you’ll find layers of Earth’s crust make up the ocean floor. These deepest layers are made up of rock and minerals.

Once they make it to the ocean, they further erode from the constant action of waves and tides. The tan color of most sand beaches is the result of iron oxide, which tints quartz a light brown, and feldspar, which is brown to tan in its original form.

The sand, and silt, used to be layers of rock, but wind and weather break it down. And then erosion washes it down into the streams and rivers, and then the rivers carry it to the sea. Some of the ocean floor is lava. Under the several kilometres of sediment, the oceanic crust is entirely volcanic in nature.

The abyssal plain is the flat area of the ocean floor. It is covered with sand, mud, and plant and animal remains. Located on this flat plain are undersea mountains called seamounts that are formed by erupting volcanoes.

Pre-expedition estimates put the Challenger Deep descent at about 90 minutes. (Animation: Cameron’s Mariana Trench dive compressed into one minute.) By contrast, some current remotely operated vehicles, or ROVs, descend at about 40 meters (130 feet) a minute, added Stern, who isn’t part of the expedition.

Human kidneys can only make urine that is less salty than salt water. Therefore, to get rid of all the excess salt taken in by drinking seawater, you have to urinate more water than you drank. Eventually, you die of dehydration even as you become thirstier.

Seawater is toxic to humans because your body is unable to get rid of the salt that comes from seawater. Your body normally gets rid of excess salt by having the kidneys produce urine, but it needs freshwater to dilute the salt in your body for the kidneys to work properly.