Astronomers have known since the 1960s that molecular clouds are the star forming "engines" of galaxies, but because of their complexity, we still lack a complete understanding of how they evolve to eventually form small, dense clumps and then stars. This is especially true for larger molecular structures called "filaments", which are dominated by dense but starless gas clumps. The PI will lead a team of undergraduate researchers to conduct the first large-scale mapping study of the internal structure, physical conditions, and gas motions for two nearby molecular clouds utilizing a variety of molecular line transitions to trace gas at intermediate to high densities with sufficient angular resolution to probe filaments and the embedded dense cores where stars should eventually form. This award will result in a valuable legacy dataset for the astronomical community and simultaneously provide a unique group-learning research experience for a large and diverse group of undergraduates, as well as engaging the public through lectures and star parties. The team will use the Arizona Radio Observatory 12m telescope (ARO 12m) to map the Taurus and Aquila Rift molecular clouds from multi-parsec filamentary scales down to sub-0.1 parsec core scales. Since there is no single tracer that can simultaneously probe these various scales and physical conditions, observations of a complementary set of line transitions capable of probing intermediate to high gas densities (HCN, H13CN, CS, SO, NH2D, c-C3H2 and CH3OH) will be used to constrain the co-evolution of molecular filaments and cores. A key intellectual merit of this award is in providing a legacy dataset to the community for studies of intermediate and high-density molecular gas in two large, nearby molecular clouds that will address fundamental questions about how mass flows within molecular clouds and help constrain the most important physical processes controlling them. The molecular gas properties revealed by these resolved observations will impact our understanding of unresolved extragalactic studies of star formation. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.