What is the water temperature at the Mariana Trench—the deepest part of the Pacific Ocean?
When dealing with ocean temperatures it's important to recognize that water temperature generally decreases as you move into deeper water.
The relatively warm surface water of oceans like the Pacific Ocean extends to depths of between 500 and 1,000 feet. Below the surface waters, the temperature drops rapidly, forming a layer called the thermocline.
The thermocline varies in thickness from about 1,000 feet to 3,000 feet. Below this point water cools more slowly. At areas like the Mariana Trench (or the Marianas Trench; both spellings are commonly found) the water temperature ranges from 34-39 degrees F (1-4 degrees C).
—The Fact Monster