Water fills a spherical tank of diameter 16 meters until the water is 4 meters deep at the lowest point in the tank. What is the diameter of the surface of the water?

Since all cross-sections of a sphere are circles we can use the fact that r^{2} = 4*(16 - 4) where r is the radius of the circular water surface.

So r = sqrt(48) = 4sqrt(3) m. Hence diameter = 8sqrt(3) m