Wandering in the Woods

Oddly enough, there was a trap described in a D&D book, in which Dwarven artificers had created hooks made out of a particular metal which - coincidentally - would bend straight when they were exposed to light. They would set up net traps on cavern ceilings, held in place with these special metal hooks, and when humans started trespassing, usually carrying torches to let them see, the nets would fall on them as the hooks straightened out.