GEOLOGY

Detailed explanation-1: -Cold water has a higher density than warm water. Water gets colder with depth because cold, salty ocean water sinks to the bottom of the ocean basins below the less dense warmer water near the surface.

Detailed explanation-2: -The ocean is very, very deep; light can only penetrate so far below the surface of the ocean. As the light energy travels through the water, the molecules in the water scatter and absorb it. At great depths, light is so scattered that there is nothing left to detect.

Detailed explanation-3: -Therefore, the deep ocean (below about 200 meters depth) is cold, with an average temperature of only 4°C (39°F). Cold water is also more dense, and as a result heavier, than warm water. Colder water sinks below the warm water at the surface, which contributes to the coldness of the deep ocean.

Detailed explanation-4: -The aphotic zone exists in depths below 1, 000 meters. Sunlight does not penetrate to these depths and the zone is bathed in darkness.