These are not wooden box mines, they probably come in a variety of sizes, but can be a coffee cup sized hollowed out wooden initiator, with a hole in the top that the lid is (gently) set in. flat wooden "pressure plate" with a small pointy stick that goes down into the hole to where the chemicals are (picture a round drink coaster with a sharpened dowel rod in the center). Stepping on this mixes the chemcials triggering the initiating blast. Attached to this are as many jugs of home made explosive as they care to apply buried beneath it.

Something "The Professor" would make to defend Gilligan's Island.