  The sub-mm sky is a unique window for probing the architecture of the
Universe and structures within it. From the discovery of dusty sub-mm galaxies,
to the ringed nature of protostellar disks, our understanding of the formation,
destruction, and evolution of objects in the Universe requires a comprehensive
view of the sub-mm sky. The current generation single-dish sub-mm facilities
have shown of the potential for discovery, while interferometers have presented
a high resolution view into the finer details. However, our understanding of
large-scale structure and our full use of these interferometers is now hampered
by the limited sensitivity of our sub-mm view of the universe at larger scales.
Thus, now is the time to start planning the next generation of sub-mm single
dish facilities, to build on these revolutions in our understanding of the
sub-mm sky. Here we present the case for the Atacama Large Aperture
Submillimeter Telescope (AtLAST), a concept for a 50m class single dish
telescope. We envision AtLAST as a facility operating as an international
partnership with a suite of instruments to deliver the transformative science
described in many Astro2020 science white papers. A 50m telescope with a high
throughput and 1$^\circ$ FoV with a full complement of advanced
instrumentation, including highly multiplexed high-resolution spectrometers,
continuum cameras and Integral Field Units, AtLAST will have mapping speeds
thousands of times greater than any current or planned facility. It will reach
confusion limits below $L_*$ in the distant universe and resolve low-mass
protostellar cores at the distance of the Galactic Center, providing synergies
with upcoming facilities across the spectrum. Located on the Atacama plateau,
to observe frequencies un-obtainable by other observatories, AtLAST will enable
a fundamentally new understanding of the sub-mm universe at unprecedented
depths.
