In hundreds of known cases, "gravitational lenses” deflect, distort, and magnify images of galaxies behind them. Lensing can magnify galaxies by factors of 10--100 times, transforming them from objects we can barely detect to bright objects we can study in detail. We have taken advantage of these natural telescopes to build Megasaura: The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas. Megasaura is an atlas of high signal-to-noise, medium spectral resolution (R~3000) spectra of 15 extremely bright gravitationally lensed galaxies at redshifts of 1.7<z<3.6. The Megasaura spectra reveal a wealth of spectral diagnostics: absorption lines from gas being blown out of the galaxy, emission lines from gas heated by newly-formed star, and photospheric absorption lines and P Cygni profiles from the massive stars that power the outflow. These will be key diagnostics for future telescopes (JWST and beyond) that with Megasaura we can study now. Indeed, Megasaura will be the definitive spectral atlas of its kind until the advent of 20–30m ground-based telescopes in the late 2020s, giving insight into the processes by which galaxies evolve over cosmic time.