  We study the low-temperature behaviour of spin ice when uniaxial pressure
induces a tetragonal distortion. There is a phase transition between a Coulomb
liquid and a fully magnetised phase. Unusually, it combines features of
discontinuous and continuous transitions: the order parameter exhibits a jump,
but this is accompanied by a divergent susceptibility and vanishing domain wall
tension. All these aspects can be understood as a consequence of an emergent
SU(2) symmetry at the critical point. We map out a possible experimental
realisation.
