Why pressure is different in the ocean?

Sometimes oceanographers will take advantage of crushing, deep-sea pressure to make decorated, shrunken Styrofoam cups as souvenirs as well as science outreach. ©Woods Hole Oceanographic Institution

Ocean waters cover about 70 percent of the planet. On average, that water is 3,800 meters (almost 2.4 miles) deep. But in the deepest spots, seawater fills a mind-boggling 11,000 meters (6.8 miles) of space above the seafloor. As anyone who has tried diving to the bottom of a deep pool knows, all that water gets heavy—fast. Extreme pressure is one reason why the ocean floor is still largely unexplored.

When a person, fish, or other marine creature swims, water presses against all sides of them. They feel equal pressure from all directions. Near the ocean’s surface, that pressure is the same as what we feel when standing on land. This amount of pressure is called an atmosphere. The term is used as a unit of measurement. It’s the pressure of Earth’s full atmosphere pressing down on us at sea level. As you travel from sea level to a higher elevation on land, atmospheric pressure drops. That’s because there’s less air above you pressing down.

Similarly, as you travel deeper in water, pressure increases. Underwater pressure is called hydrostatic pressure. (Hydro means water. Static means at rest.) The more water you have above you, the more it pushes against you. But water doesn’t only push down from above. It pushes in from all sides and even from below. That upward force is called buoyancy. It’s what keeps you afloat in water.

Imagine that a diver swims straight down from the surface. As they go, their ears clog. It’s the same feeling you get when you drive up into the mountains. This happens because there’s more pressure on one side of the eardrum than the other. The diver has to swallow or wiggle their jaw to make the pressure equal on both sides. This is what happens when you “pop” your ears.

When the diver reaches 10 meters (33 feet), the pressure is double what it was at the surface. For every 10 meters of water, hydrostatic pressure increases by one atmosphere. At the average ocean depth (3,800 meters), pressure on the sea floor is a whopping 380 times greater than it is at the surface. In the deepest trenches, it’s 1,100 times greater!

All that pressure causes serious problems for people and other air-breathing animals. Our lungs fill with air, and we have air-filled spaces in our heads. Too much pressure would collapse those spaces, crushing us. Animals adapted to deep-ocean life don’t have air pockets in their bodies.

Some marine animals travel between deep ocean and the surface. Sperm whales are the deepest diving mammals. They breathe air. But they have collapsible ribs and lungs that allow the whales to handle the pressure without causing damage.