  A weak topological insulator (WTI) bears, generally, an even number of Dirac
cones on its surface; they are susceptible of doubling, while on the surface of
a certain orientation it shows no Dirac cone. On this "dark" surface of a WTI,
we predict the existence of a single pair of isolated 1D perfectly conducting
channels that forms either a closed loop or a segment of a line. The former is
associated typically with a single atomic-layer-thick island formed on the dark
surface, while the latter is shown to be the consequence of a pair of crystal
(screw) dislocations terminating on the dark surface.
