  We study macroscopic quantum tunneling of interfaces separating normal and
superconducting regions in type-I superconductors. Mathematical model is
developed, that describes dissipative quantum escape of a two-dimensional
manifold from a planar potential well. It corresponds to, e.g., a
current-driven quantum depinning of the interface from a grain boundary or from
artificially manufactured pinning layer. Effective action is derived and
instantons of the equations of motion are investigated. Crossover between
thermal activation and quantum tunneling is studied and the crossover
temperature is computed. Our results, together with recent observation of
non-thermal low-temperature magnetic relaxation in lead, suggest possibility of
a controlled measurement of quantum depinning of the interface in a type-I
superconductor.
