In winter it is easy to see how much damage ice causes when it forms underground, but there may be more to the process than meets the eye. Robert Style of the University of Oxford and colleagues have created a physically intuitive model that goes a long way towards explaining what goes on, and in particular how "ice lenses" form, pushing up large regions of ground. Naively it might seem that this is a result of ice taking up more space than water, but similar behaviour is found with liquids that do not shrink on freezing.

A key insight is a new concept called "geometrical supercooling". The idea is that ice crystals can be prevented from growing through more water freezing if they are confined and under pressure. More and more water can be drawn up into a supercooled region via thermomolecular pressure gradients or "cryosuction", until the soil gives way and an ice lens forms. In this case, it is not just that water expands on freezing, but crystal growth that does the pushing.