>>50
You mean a shaped charge. Basically, they dug out a cylinder from the ground, placed the nuke at the bottom, and on the top of the cylinder, at ground level, they placed a cone with its tip pointing towards the nuke. Can't remember what the cone is made out of, but regular shaped charges have cones made out of copper.
Not sure if this is something they thought of or it's something they actually made. I heard the americans accidentally sent a sewer lid to space with a nuke, to similar effect, but I'm not sure whether or not this happened.