A cone-shaped mountain has its base on the ocean floor and has a height of 8000 feet. The top \dfrac{1}{8} of the volume of the mountain is above water. What is the depth of the ocean at the base of the mountain, in feet?
Answer Choices
A. 4000
B. 2000(4-\sqrt{2})
C. 6000
D. 6400
E. 7000