Subsidences which occur within, or near, the outcrop of the Chalk are due to the collapse either of solution features or of old mine workings. However, only the latter are considered here. Mine workings in the Chalk extend back into the distant past, the most ancient workings being those which Stone Age man excavated in his quest for flint. The collapse of old mine workings is unpredictable and, to make the situation worse, most old workings are unrecorded and are therefore a potential hazard in areas scheduled for development. The investigation of such potentially hazardous ground conditions, especially the detection of sub-surface cavities, is by no means a simple task although geophysical methods of exploration are being continually improved. Remote sensing and aerial photography have also proved useful in recognizing the surface expression of mine workings. In addition, hazard mapping techniques have been developed which attempt to identify problem ground conditions. Two case histories of mine collapse at Bury St Edmunds, Suffolk and Grays, Essex are described.