  We report on the experimental realization of a conservative optical lattice
for cold atoms with sub-wavelength spatial structure. The potential is based on
the nonlinear optical response of three-level atoms in laser-dressed dark
states, which is not constrained by the diffraction limit of the light
generating the potential. The lattice consists of a 1D array of ultra-narrow
barriers with widths less than 10~nm, well below the wavelength of the lattice
light, physically realizing a Kronig-Penney potential. We study the band
structure and dissipation of this lattice, and find good agreement with
theoretical predictions. The observed lifetimes of atoms trapped in the lattice
are as long as 60 ms, nearly $10^5$ times the excited state lifetime, and could
be further improved with more laser intensity. The potential is readily
generalizable to higher dimension and different geometries, allowing, for
example, nearly perfect box traps, narrow tunnel junctions for atomtronics
applications, and dynamically generated lattices with sub-wavelength spacings.
