An underwater bubble is well known to grow in response to a strong negative acoustic pulse and then collapse because of inertial forces. Here it is shown that adding an auxiliary positive pulse after collapse begins intensifies the collapse. The negative-then-positive pulse sequence is produced by two ellipsoidal reflectors, each with an electrical spark at its near focus f1 and beamed so that they share a common second focus f2. The negative pulse is produced by a polyurethane (pressure release) ellipsoid, the positive by a brass (rigid) ellipsoid. A timing circuit controls the delay between the pulses. Cavitation is recorded by pitting (caused by bubble collapse) of an aluminum foil membrane, which is centered at f2 and lies coplanar with the two crossed beams. When the brass reflector is fired alone, a narrow path of ∼1-mm diameter pits appears. Firing the polyurethane reflector alone yields more widespread, ∼0.1-mm diameter pits. When both are fired, a pitted X pattern shows the position of the two beams. If the delay between the two firings is 2–6 μs, the intersection of and the centerline between the paths erupts with deep pits. [Work supported by ONR.]