  The transient cavity dynamics during water entry of a heavy, non-rotating
sphere impacting a rotating pool of liquid is studied experimentally,
numerically, and theoretically. We show that the pool rotation advances the
transition of the cavity type - from deep seal to surface seal - marked by a
reduction in the transitional Froude number. The role of the dimensionless
rotational number $\mathcal{S} \equiv \omega R_0/U_0$ on the transient cavity
dynamics is unveiled, where $R_0$ is the sphere radius, $\omega$ the angular
speed of the liquid, and $U_0$ the impact velocity. The rotating background
liquid has two discernible effects on the cavity evolution. Firstly, an
increase in the underwater pressure field due to centripetal effects, and
secondly a reduction in the pressure of airflow in the cavity neck near the
water surface. The non-dimensional pinch-off time of the deep seal shows a
robust 1/2 power-law dependence on the Froude number, but with a reducing
prefactor for increasing $\omega$. Our findings reveal that the effects of a
rotating background liquid on the water entry can be traced back to the subtle
differences in the initial stage splash and the near-surface cavity dynamics.
