According to a 2011 report from the Council for Geoscience, 50 towns around the country -- including parts of Johannesburg and Pretoria -- sit fully or partially on dolomite, a rock that is predisposed to sinkhole formation. The three major methods of sinkhole formation are the lowering of the water table, water leaking from poorly managed infrastructure and surface water drainage.
That first method initially sparked my interest, as mining dewatered the Far West Rand, dropping the area's water level. The sinkholes in the above photo formed a little more than a year ago, likely caused by pipes dripping over a dewatered area.