  Very recently, it was pointed out that when basic ideas of the Swampland
program are combined with the cosmological hierarchy problem (i.e. the
smallness of the dark energy in Planck units) and together are confronted to
experiment lead to the prediction of the existence of a single extra-dimension
(dubbed the dark dimension) with characteristic length-scale in the micron
range. We propose a feasible test procedure to probe the dark dimension using
the highest energy cosmic rays.
