Surprisingly large column densities of H_3~+ have been detected using infrared absorption spectroscopy in seven diffuse cloud sight lines (Cygnus OB2 12, Cygnus OB2 5, HD 183143, HD 20041, WR 104, WR 118, and WR 121), demonstrating that H_3~+ is ubiquitous in the diffuse interstellar medium. Using the standard model of diffuse cloud chemistry, our H_3~+ column densities imply unreasonably long path lengths (～1 kpc) and low densities (～3 cm~(-3)). Complimentary millimeter-wave, infrared, and visible observations of related species suggest that the chemical model is incorrect and that the number density of H_3~+ must be increased by 1-2 orders of magnitude. Possible solutions include a reduced electron fraction, an enhanced rate of H_2 ionization, and/or a smaller value of the H_3~+ dissociative recombination rate constant than implied by laboratory experiments.
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