# Trench Dive

Some parts of the greatest ocean depths, the Marianas Trench, are around 11,000 meters deep. This means that the top of the tallest mountain on Earth, Mount Everest, if placed in the trench would be over 2,000 meters below the surface of the ocean. Scientists send submersibles to these great depths to determine characteristics of the seawater and life that exists there. Are you ready to go deep on this watery adventure?

### What is pressure?

Pressure is defined as the force per unit area. For a given area, pressure increases with increasing force. For a given force, pressure decreases with increasing area. For example, a 5-pound book resting on a person's arm would not be too painful or cause much damage; however, if the same book was placed on the arm with a needle between the book and the arm, the increase in pressure due to the reduction in area would cause the person much pain.

### What is density?

Density can be thought of as the compactness of a material. The more compact the material, the higher the material's density. Density is defined as the mass of an object divided by its volume. Common units of density are grams per milliliter or grams per cubic centimeter.

### What is salinity?

Salinity refers to the amount of salt, primarily sodium chloride or table salt, in water. Fresh water typically has salinity around 0.05%, while ocean water's average salinity is about 3.5%; the higher the percentage value, the more salt is dissolved in the water.

### What are the layers of the ocean?

Ocean water is divided into four different layers depending on the depth and amount of sunlight reaching the layer. The uppermost layer at the surface of the ocean is called the Epipelagic zone or the sunlight zone, and extends down to a depth of around 200 meters. The next layer down is called the Mesopelagic zone or twilight zone with a depth from 200 - 1,000 meters. The Bathypelagic zone, also called the midnight zone, extends from 1,000 to 4,000 meters. The deepest zone, called the Abyssopelagic zone, goes from 4,000 to a depth of 6,000 meters.

### How does pressure vary with ocean depth?

In a fluid, the pressure increases with increasing depth. The equation that is used to calculate the pressure at a given depth is: pressure equals fluid density times acceleration due to gravity times the depth of the fluid. The pressure in a fluid also acts equally in all directions at a given depth. This means that a diver lying flat at a given depth has the same amount of pressure acting down on his back as pushes on his nose or on his belly.

### How does temperature vary with ocean depth?

The temperature of ocean water varies with depth. Since sunlight is the primary heating source, depths that receive little sunlight would be expected to be the coldest regions of the ocean. Ocean water temperature decreases gradually in the Epipelagic zone, then falls off sharply in the Mesopelagic zone, and remains a fairly constant 4 °C in depths below that zone.

### How does water density vary with ocean depth?

The density of ocean water at the surface averages about 1.025 g/mL. From the surface to around a depth of 1,000 meters, the density of the ocean water increases steadily to about 1.028 g/mL. Beyond this depth, the density of the water remains fairly constant at this same value.

### How does salinity vary with ocean depth?

Ocean surface water generally has the highest salt concentration (about 3.50%) of any depth. From the surface to a depth around 600 meters, the salinity decreases steadily to about 3.40%. The water's salinity then increases very slightly and remains fairly constant at 3.45% all of the way to the deepest trenches.