The continental shelf is an undersea extension of a continent that can stretch a great distance out to sea in some cases. A continental shelf is not necessarily flat. It may be marked with deep valleys and other geological features that may continue out to the abyssal plain, the deepest part of the ocean. The shelf is also covered in a thick layer of nutrient rich sediment that is deposited by rivers and streams. These nutrients support varied and diverse marine life including seaweeds, fish, and many microscopic organisms, like dinoflagellates and other plankton.