The paper reports an experimental investigation by LDV and high-speed camera visualization, of the fluid motion underneath water drop impacting onto a deep pool of the same liquid for different drop impact velocities in the low Weber number regime in isothermal conditions. Vortex ring structures could be very sensitive of pool and ambient temperature, which may both affect the crater evolution (through the influence on surface tension) and directly the vortex ring evolution by changing the Reynolds number through the variation of the viscosity. The measured velocity field is compared to the visualization results to clarify some peculiar regimes of crater evolution (like bubble entrapment, vortex ring motion, etc.) during the expansion and the recoiling phase.