An area around Horndean in SE Hampshire, England, has been studied to determine the reasons for an unusually high density of sinkholes related to dissolution of the underlying chalk. Ground investigations and excavations for a variety of engineering and construction projects in this area have provided detailed information that gives an insight into their formation and development. Contouring of the chalk surface indicates the presence of a north–south-trending palaeovalley, beneath Tertiary cover deposits, to the south of Horndean. The significance of this feature to chalk karstification and continuing sinkhole development is discussed, along with the significance of other geological factors, such as fracture zones in the chalk and the nature and distribution of cover deposits. This forms the basis of a risk assessment of ground subsidence in this area, with particular reference to collapse. The engineering significance of the sinkholes and the design of ground investigations are discussed in the light of experience gained from past projects in this area and the established risk of pollution of the chalk aquifer