$\pi \times r^2\times d$,

where

$r=$ radius (half the diameter)

$d=$ depth

$\pi =3.141\dots$ (goes on forever).

The reason I know this is that $\pi \times r^2$ is the area of the disc and then I multiplied this by depthto get the volume.

If the pond had been square-shaped then I would have multiplied the area of a square by depth.

Is that okay?