You are always under pressure. Air presses down on you at all times at 14.5 pounds per square inch, also called one bar or one atmosphere. Human beings can withstand 3 to 4 atmospheres of pressure, or 43.5 to 58 psi. Water weighs 64 pounds per cubic foot, or one atmosphere per 33 feet of depth, and presses in from all sides. The ocean's pressure can indeed crush you.
One reason you couldn't withstand the pressure of the ocean's depths without serious protection is the difference in volume between pressurized air and pressurized water. Molecules in air are spread out -- you can squish them together quite a bit without making the air in a given space weigh very much more. Water doesn't squish -- a cubic foot of liquid water always weighs 64 pounds, and 64 pounds of liquid water always takes up one cubic foot of space. Deep water would squeeze the air in your body, especially in your sinuses and lungs. Since air is compressible, these spaces would eventually collapse.
Gas: Nothing to Laugh About
Water pressure would eventually crush your other soft tissues as your depth increased, but you would die of two effects of gas compression and expansion long before this happened. The first is nitrogen narcosis. Air is made up of different proportions of several different gasses. Scuba equipment lets humans dive longer and deeper by providing compressed air under the same pressure as the surrounding water to prevent lung collapse. However, compressed air has more of each component gas, including nitrogen. Your body doesn't use nitrogen for anything. Under normal conditions, it's simply exhaled. But when you breathe pressurized air, you're breathing much higher levels of nitrogen than normal. The extra nitrogen dissolves in the blood and interferes with normal nerve impulses. At depths beginning around 100 feet, it gives a sense of euphoria or drunkenness. By 300 feet, it causes loss of consciousness and death.
You'll face the second danger of excess nitrogen as you rise back to the surface. All the gas in a diver's blood is compressed by the surrounding water. As he rises, this gas expands. Ascent must be slow and controlled to allow gradual expansion, travel to the lungs and exhalation. If an ascent is too fast, nitrogen bubbles may form inside the diver's capillaries and block blood flow. This causes terrible pain known as "the bends." It can also cause blindness, paralysis, organ impairment and death. When gas in the lungs expands too fast, it can cause arterial gas embolism. This happens when the tiny air sacs in the lungs, called alveoli, burst, releasing gas bubbles into the bloodstream traveling to the brain. These bubbles can cause stroke-like symptoms and death.
The Outer Limits
Deep-diving marine mammals have special adaptations to resist sudden pressure changes. Depth dwellers have adaptations to live under constant pressure. Human beings are bound by our physiology and technology. Past depths of about 99 feet -- 4 atmospheres of pressure -- reinforced atmospheric suits are necessary. These allow the person inside to experience a single atmosphere of pressure up to depths of around 2000 feet. Exploration from about 2000 feet to the ocean's maximum of 36,200 feet requires special equipment, predominantly unmanned. The challenges involved in getting down there mean the ocean's depths -- 60 per cent of Earth's ecosystem -- are still largely unknown.