  The growth of crystals confined in porous or cellular materials is ubiquitous
in Nature and industry. Confinement affects the formation of biominerals in
living organisms, of minerals in the Earth's crust and of salt crystals
damaging porous limestone monuments, and is also used to control the growth of
artificial crystals. However, the mechanisms by which confinement alters
crystal shapes and growth rates are still not elucidated. Based on novel
\textit{in situ} optical observations of (001) surfaces of NaClO$_3$ and
CaCO$_3$ crystals at nanometric distances from a glass substrate, we
demonstrate that new molecular layers can nucleate homogeneously and propagate
without interruption even when in contact with other solids, raising the
macroscopic crystal above them. Confined growth is governed by the peculiar
dynamics of these molecular layers controlled by the two-dimensional transport
of mass through the liquid film from the edges to the center of the contact,
with distinctive features such as skewed dislocation spirals, kinetic
localization of nucleation in the vicinity of the contact edge, and directed
instabilities. Confined growth morphologies can be predicted from the values of
three main dimensionless parameters.
