When a ball is dropped in fine very loose sand, a splash and subsequently a jet are observed above the bed, followed by a granular eruption. To directly and quantitatively determine what happens inside the sand bed, high-speed X-ray tomography measurements are carried out in a custom-made set-up that allows for imaging of a large sand bed at atmospheric pressures. Herewith, we show that the jet originates from the pinch-off point created by the collapse of the air cavity formed behind the penetrating ball. Subsequently, we measure how the entrapped air bubble rises through the sand, and show that this is consistent with bubbles rising in continuously fluidized beds. Finally, we measure the packing fraction variation throughout the bed. From this we show that there is (i) a compressed area of sand in front of and next to the ball while the ball is moving down, (ii) a strongly compacted region at the pinch-off height after the cavity collapse and (iii) a relatively loosely packed centre in the wake of the rising bubble.