stinging nettles, a.k.a. "Hellweed", leafy knee-high plant which grows profusely in shady, damp places-- i.e. most cave areas. Tiny hairs on the underside of the leaves secrete an oil which gives the skin a burning sensation. The sensation is delayed 10-20 seconds, so those who are unwary usually end up in a really thick patch before realizing it. Good for tricking novices ("is that an entrance over there?") Nettles are reason to avoid caving in the summer. Nasty.