  We show that a quantum-mechanical N=2 supersymmetry is hidden in 4d mass
spectrum of any gauge invariant theories with extra dimensions. The N=2
supercharges are explicitly constructed in terms of differential forms. The
analysis can be extended to extra dimensions with boundaries, and for a single
extra dimension we clarify a possible set of boundary conditions consistent
with 5d gauge invariance, although some of the boundary conditions break 4d
gauge symmetries.
