As cosmic rays traverse the interstellar medium (ISM) they interact with the ambient gas in various ways. These include ionization of atoms and molecules, spallation of nuclei, excitation of nuclear states, and production of pions among others. All of these interactions produce potential observables which may be used to trace the flux of cosmic rays. One such observable is the molecular ion H 3 + -produced via the ionization of an H 2 molecule and its subsequent collision with another H 2 -which can be identified by absorption lines in the 3.5-4 μm spectral region. We have detected H 3 + in several Galactic diffuse cloud sight lines and used the derived column densities to infer ζ 2 , the cosmic-ray ionization rate of H 2 . Ionization rates determined in this way vary from about 7×10 -17 s -1 to about 8×10 -16 s -1 , and suggest the possibility of discrete sources producing high local fluxes of low-energy cosmic rays. Theoretical calculations of the ionization rate from postulated cosmic-ray spectra also support this possibility. Our recent observations of H 3 + near the supernova remnant IC 443 (a likely site of cosmic-ray acceleration) point to even higher ionization rates, on the order of 10 -15 s -1 . Together, all of these results can further our understanding of the cosmic-ray spectrum both near the acceleration source and in the general Galactic ISM.