Sentence:  On certain days, when hot currents shimmered off Oyster's Reef, we would detect the chalk-dust of the mullock heaps, acrid; or, from the opal mines themselves, the ghastly <WSD>fug</WSD> of the tunnels and shafts. 
Meaning of the word marked with <WSD> tags:  A heavy, musty, and unpleasant atmosphere, usually in a poorly-ventilated area.