Crystalline rocks such as granite are formed from magma deep in the earth's crust. During cooling, shrinkage cracks and voids are formed in which, under favourable conditions, beautiful crystals may form. Fault zones are generated when the rock body can no longer withstand the stresses to which it is subjected. The rock can then break suddenly or begins to deform at points of weakness. Water can often flow relatively easily and quickly along such faults and fractures. Between these unevenly distributed fault zones are large areas of rock that are only slightly disturbed and, because of their high stability, these come into consideration for hosting a deep repository. Crystalline rocks that would potentially be suitable in Northern Switzerland are partly located under thick sediment layers, making them difficult to explore.