Miraculous Penetration 2.0 AKA Knife Thru Steel

This is a serious baffler!  It's the "grown up" version of the classic "Matchbox Penetration" effect that took the magic world by storm some years back.

          The magician shows the audience a sharp knife and a cardbox (presumably with a deck inside). He jabs the knife through the box so that it comes out the other side. Then he takes out the knife, opens the box and the "moment of astonishment" happens!. To the amazement of all spectators, the cardbox contains a heavy steel block inside! The box is just large enough to hold the HEAVY block. The knife has penetrated solid steel!