system
Given the following post, respond as if you're commenting on Reddit:
human
How cold is the water at the bottom of the deepest parts of the ocean?

I know water is most dense at 4 degrees Celsius so is the water at the bottom of the ocean 4 degrees Celsius or does the pressure of that depth have an effect on the temperature or am I on the wrong track entirely?
gpt
Density rules. The heaviest, or most dense, water will always be on the bottom. Pure water is densest at 4 C . To make it heavier, it has to get saltier. As the surface of the Arctic ocean freezes, the left-behind part is very salty water at 3 to 4 C. This North Atlantic Deep Water sinks to the bottom and flows worldwide. Warmer less salty water sits on top of it.

In the Antarctic, there is no warmer fresher water to sit on top of it, so NADW is exposed at the surface. When fresh water freezes out at the surface, this already salty and cold water gets even saltier and can cool to about -0.8C before sinking. This is Antarctic Bottom Water - the deepest, coldest water layer in the world.

An interesting point is that the Marianas Trench, the deepest spot in the oceans, is only at about plus 2 C. This is because it is completely surrounded by water shallow enough (at only a mile or so deep) that AABW would have to flow uphill to get to it.