The dynamic plunging behavior of a subcooled water jet into a pool of molten lead-bismuth alloy, visualized and measured using high-frame-rate neutron radiography, is described. It is shown that the interactions between water and heavy melt in this geometry differ in several aspects from those in melt injection into water investigated extensively in relation to fuel-coolant interactions. The maximum depth of jet penetration is limited by the buoyancy on and the onset of bulk boiling in the water which has collected in a "cavity." The bulk boiling starts as the subcooled water supply to the cavity becomes limited due to pinching instabilities in the upper region of the cavity. This leads to a transition to the final steady state.