The cool, molecular phase of the interstellar medium is the fuel that enables the formation of new stars. In the early universe, large gas reservoirs dominated the baryonic mass of galaxies and enabled a cosmic star formation density that peaked at 10 times the current value. While the light from young stars has made it possible to trace the star formation itself, observing the molecular gas itself is much more difficult. This phase, typically traced by CO emission, has only been observed at high redshift in the most massive objects, while normal galaxies are nearly inaccessible to even the most sensitive radio telescopes. The technique of “intensity mapping,” which measures the aggregate molecular emission from the three-dimensional distribution of galaxies, provides a tool to detect the faint signal of the molecular ISM and chart its history across cosmic time. I will describe an intensity mapping experiment that targets CO emission from the peak of cosmic star formation, the CO Power Spectrum Survey (COPSS). Through the combination of archival and targeted observations with the CARMA interferometer, COPSS has constrained the CO power spectrum at z~3 for the first time. I will review these measurements, their implications for the distribution of molecular gas in galaxies, and the prospects for related measurements in the future.