Work to identify suitable geological environments for deep disposal of radioactive waste began in the 1960s and, by the 1980s, many possibilities had been defined, namely bedded salt,30 salt domes,31 unsaturated rocks,32 basement under sedimentary cover (BUSC),3 3 hard rock in low relief terrain, deep sedimentary basins, seaward dipping and offshore sediments, low permeability formations and small islands in the sea.34 In all these cases the underlying aim is either to prevent water contact with the waste (as with salt and unsaturated sites) or else to minimise its effects. BUSC is an interesting (and not uncommon) example where the higher permeability of the sedimentary cover means that groundwater flow is directed away from the less permeable basement rock, making the latter a more suitable host for a repository. Further, the permeability of sedimentary rocks is often anisotropic, being higher parallel to the bedding plane. This effect will also tend to direct the flow into the upper layers.