Imagine a place only accessible by deep-sea submersible. A place that's so cold only the smallest microbes and microorganisms can survive. A place where water exhibits such tremendous amount of pressure that it equals 16,000 pounds per square inch. At almost 7 miles in depth, the deepest point of the ocean is called the Marianas Trench. Located in the Pacific Ocean, this trench is part of a continuous trough on the ocean floor that runs over 1,550 miles long.
How is the trench measured?
The basic problem with measuring the deepest point of the ocean is simply that it's inaccessible to humans. To measure depth under such extreme pressure, scientists use a device called an echo sounder. Mounted on a ship or submersible, this machine sends out a sound wave that heads directly down to the ocean floor. Scientists measure the amount of time required for this sound wave to travel from the water's surface to the ocean floor and back again.
Exact measurements differ. In 1995, a Japanese probe measured the deepest point of the Marianas, called the Challenger Deep, at 35,798 feet. Other measurements have indicated a depth of 36,160 feet. Despite this slight discrepancy, we're still talking about an underwater V that is almost 7 miles deep. Challenger Deep was named after the British survey ship Challenger II that discovered it in 1951.
For perspective, let's look at the Atlantic Ocean. The deepest point is 28,232 feet located in the Puerto Rico Trench. That means that the Atlantic Ocean's deepest point is 5.34 miles deep as opposed to the Pacific's deepest measurement at 6.77 miles. The Pacific Ocean's deepest point is almost a 1 miles deeper.
On the earth's surface, the highest point above sea level is Mount Everest, reaching a soaring 29,035 feet. The Challenger Deep in the Marianas Trench is still over 1 miles deeper than Mount Everest. Of course, things are in reverse in this case. If you took Mount Everest, turned it upside down and stuck it inside the Marinas Trench, there would still be 1 mile of space available.
So why is it so deep?
In many oceans, deep trenches fill with sediments that wash off the slopes of the trough. This decreases the trench's depth, much filling it up like a hole dug in a sandbox. Some sediment accumulates in the Marianas but much stronger forces are at work to form the deepest point of the ocean.
The Marianas Trench lies in an active area of oceanic plate movement. Called a subduction zone, the trench was actually formed as an oceanic plate slipped under a continental plate. Oceanic plates are heavier so the continental plate rides on top of it, forming a deep V-shaped feature on the underwater landscape.
Does anything live there?
Using a deep-sea, underwater submersible, scientists collected soil samples from some of the deepest parts of the Marianas Trench. Surprisingly, they discovered life at such great depths of the ocean. Single celled organisms called foraminifera live in the trench. These tiny organisms occupy the lower form of the food chain that exists in the ocean.