  Sulci are localized furrows on the surface of soft materials that form by a
compression-induced instability. We unfold this instability by breaking its
natural scale and translation invariance, and compute a limiting bifurcation
diagram for sulcfication showing that it is a scale-free, sub-critical {\em
nonlinear} instability. In contrast with classical nucleation, sulcification is
{\em continuous}, occurs in purely elastic continua and is structurally stable
in the limit of vanishing surface energy. During loading, a sulcus nucleates at
a point with an upper critical strain and an essential singularity in the
linearized spectrum. On unloading, it quasi-statically shrinks to a point with
a lower critical strain, explained by breaking of scale symmetry. At
intermediate strains the system is linearly stable but nonlinearly unstable
with {\em no} energy barrier. Simple experiments confirm the existence of these
two critical strains.
