I believe either a Basin or a Valley.
A large depression in the land or ocean floor is typically known as a basin or a trough. These depressions can form through various geological processes such as tectonic activity, erosion, or volcanic activity. Basins and troughs can vary in size and shape, and are often associated with the accumulation of sediments.
trench
This section of the ocean from floor to surface is called the water column.
The ocean's surface
Challenger Deep.
water column
Depth is a measure of distance between the surface and the ocean floor.
70% of the size of the Earth.
The crust.
Nemo lives in the coral reef which is near the surface of the ocean. So the drop off is the part to the ocean floor. Remember the 3 parts of the ocean: The surface, the mid-section, and the ocean floor!
TRUE
the tectonic plates under the earths surface