These were not designed to directly penetrate defences, though they could do this (for example, the Valentin submarine pens had ferrous concrete roofs 4 meters thick which were penetrated by two Grand Slams on 27 March 1945), but rather to penetrate under the target and explode leaving a camouflet (cavern) which would undermine foundations of structures above, causing it to collapse, thus negating any possible hardening.