In the center of the lid, there was a  pressure plate of thick green glass which resembled a glass stopper. In the center of the mine, the pressure plate sat on top of a glass igniter that was embedded in  of TNT. Only pressure on the plate caused an explosion and pressure on the cardboard side of the mine would not set it off.
The center of the lid featured a thick green glass pressure plate that resembled a stopper. Inside the mine, this plate sat atop a glass igniter embedded in TNT. An explosion occurred only when pressure was applied to the plate, and pressing on the cardboard side did not trigger it.