I refer to the article on the Guatemala City sinkhole (NCE last week). As a student at the University of the Witwatersrand in the 1960s, I can recall several lectures given by Professor Jere Jennings on the sinkholes in the karstic formations underlying Carletonville in the West Rand.
He and others investigated the problem extensively and concluded that the main cause was the dewatering of vast caverns in the dolomite by mining activities.
This led to an increased vertical flow of water eroding the overlying soils from the bottom up into the empty caves, eventually leading to catastrophic collapse at the surface and the formation of enormous sinkholes - very similar to the picture in NCE.
Gwynne Owen-Smith (M), IMS engineer, Carillion CTRL IMC, Ashford, Kent.