  We propose a superconducting instability where microscopic supercurrent loops
form spontaneously within a unit cell at the superconducting transition
temperature with only uniform, onsite and intra-orbital singlet pairing. As a
result of the circulating currents time-reversal symmetry is spontaneously
broken in the superconducting state. Using Ginzburg-Landau theory, we describe
in detail how these currents emerge in a toy model. We discuss the
crystallographic symmetry requirements to realize such a state and show that
they are met by the Re6X (X=Zr, Hf, Ti) family of time-reversal symmetry
breaking, but otherwise seemingly conventional, superconductors. We estimate an
upper bound for the resulting internal fields and find it to be consistent with
recent muon-spin relaxation experiments.
