The coldest and densest structures of gas and dust in the Universe have unique spectral signatures across the (sub-)millimetre bands (ν ≈ 30 − 950 GHz). The current generation of single dish facilities has given a glimpse of the potential for discovery, while sub-mm interferometers have presented a high resolution view into the finer details of known targets or in small-area deep fields. However, significant advances in our understanding of such cold and dense structures are now hampered by the limited sensitivity and angular resolution of our sub-mm view of the Universe at larger scales. In this context, we present the case for a new transformational astronomical facility in the 2030s, the Atacama Large Aperture Submillimetre Telescope (AtLAST). AtLAST is a concept for a 50-m-class single dish telescope, with a high throughput provided by a 2 deg -diameter Field of View, located on a high, dry site in the Atacama with good atmospheric transmission up to ν ∼ 1 THz, and fully powered by renewable energy. We envision AtLAST as a facility operated by an international partnership with a