The unique weapon that allowed the airborne engineers to neutralise the casemates of Eben Emael were fifty-six 50kg (110lb) and 12.5kg (27.5lb) Hohlladung or "Hollow Charges". Also known as shaped charges they consisted of a hollow sphere of TNT with carrying handles. When a shaped charge was detonated it produced a convergent shock wave and so a directional explosion. If the inside of the charge was lined with a metal like copper this made it even more effective. The explosive gas and jet of molten metal could penetrate concrete or armour. The principle of shaped charges had been discovered by an American, C.E. Munroe, in 1 888 and was known to engineers as the "Munroe Effect". In Germany work was undertaken by Foerster and Neumann. Hollow charges would form the basis of the warheads of rocket propelled anti-tank weapons during and after World War II.