You fire a femtosecond near-infrared laser pulse with enough power into the air, and air starts to lase in the UV. After over a hundred of definitive papers on this subject, we still do not quite know how it works, but I will share with you what we think we have learned.
The physics of air lasing is as intricate as it is beautiful, and may involve forbidden transitions becoming allowed (usually by mistake), lasing without inversion (without even trying for it), and real population inversion -- the latter is really hard to come by, but we hope to achieve it (numerically) by October 8, 2021 and we think it was achieved experimentally by D. Kartashov and colleagues in .
 Kartashov, D., et al. "Transient inversion in rotationally aligned nitrogen ions in a femtosecond filament." High Intensity Lasers and High Field Phenomena. Optical Society of America, 2014.