Hi Mike,

You didn't specify the shape of the pool but I assume the surface is a circle of radius 24 feet.

I see the hole as a cylinder with base a circle of diameter 24 feet and height 1 inch, and a cone with a base a circle of diameter 24 feet and height 9 inches.

The two important facts here are that the volume of a cylinder is the area of the base tomes the height and the volume of a cone is one third the volume of the cylinder with the same base and height. Thus the volume of the cylindrical top layer is $\pi \; r^2 h$ where $r$ is the radius, 12 feet, and $h$ is the height, 1 inch. The volume of the cone is $\large \frac13 \normalsize \pi \; r^2 h$ where $r$ is again 12 feet but $h$ is 9 inches.

Now you can use our volume calculator. First use it to find the volume of the top layer. Next use it to find the volume of a cylinder with radius 12 feet and height 9 inches and take a third of the result to find the volume of the cone.

I hope this helps,
Penny