As a physical oceanographer, I'm used to thinking about the deep ocean. Figure 1 shows a side view of temperature distribution in the top kilometer of the Pacific Ocean at a latitude of about 11.33 degrees N and across the Pacific's entire width. Colors represent different temperatures, with numbers showing some values (Centigrade). Divers most often stick to the top few tens of meters of the ocean (the warm part shown in red at the top of the figure). However, there are interesting patterns of temperature reaching down hundreds of meters. Notice the sharp change in temperature at around 100-200 meters depth. Often a change in temperature marks a deep ocean current which can carry many times the amount of water of all the rivers of the Earth combined.

While deep by human standards, depths that physical oceanographers are usually interested in are only a small fraction of the distance James Cameron travelled. Figure 2 expands the view to the entire depth of the ocean. Most of the ocean consists of a mass of water that is all just a little above freezing and which reaches down to five or six kilometers. Deep trenches like the Marianas Trench only occur in a few locations in the ocean but are about twice as deep as most of the sea. The Marianas Trench can be seen at about longitude 140 in the figure.

Standard ocean atlases don't even include temperature measurements below 5500 meters. In fact it becomes difficult to lower an instrument down that far on a steel wire. Japanese scientists recently had a custom-made titanium wire made to lower a thermometer to make measurements (Kaira et al, 2005, Journal of Oceanography). Water in the trench has a similar temperature to that in the rest of the deep ocean.

Deep sea trenches tend to be long and narrow and occur near island chains or continents. The left panel of Figure 3 shows a map of the Marianas Trench (blue) next to the shallow ridge that holds the Marianas Islands (red). A cross-section from south to north (black dots at bottom of map) reveals the V-shape of the trench (Figure 3 right panel).