Sentence:  By the mid-1940s members of local <WSD>grottos</WSD> (regional clubs of cavers within the National Speleological Society) were exploring Porcupine Cave (Bloch, 1946). 
Meaning of the word marked with <WSD> tags:  A local organization of cavers that typically organizes trips to caves and provides information and training for caving; a caving club.