The following table lists what percentage of Earth's surface lies at different elevations (in Km)

>5 (Km) 0.1%
4-5 0.4
3-4 1.1
2-3 2.2
1-2 4.5
0-1 20.8 (above sea level)
1-0 8.5 (below sea level)
2-1 3.0
3-2 4.8
4-3 13.9
5-4 23.3
6-5 16.4
<6 1.0

Source (Strahler, Physical Geography, 1975)

The deepest spots in the ocean are the oceanic trenches, the deepest (Marianas Trench, or Challenger Deep) is almost 11 Kilometers below sea level). However, the great effort and expense to design a craft to operate at these depths (which comprise less than one percent of the earth's area), may be counter productive. Are we better off using those same resources to build more submersibles to better explore the other, shallower 99%?

Of course, we have to go there, it goes without saying. But this kind of analysis should also be applied to space exploration. Resources are limited, we can't do it all. Not all at once. We have to prioritize and make hard choices. Granted, the Challenger Deep may be a more worthwhile destination than most of the abyssal plain, but there may be things in the big ocean basins we can't afford to miss either.