Long-lived particles are predicted by many extensions of the Standard Model and have been gaining interest in recent years. In this paper the original proposal is presented for AN Underground Belayed In-Shaft (ANUBIS) detector that substantially extends the sensitivity to particle lifetimes by instrumenting the existing service shafts above the ATLAS or CMS experiments with tracking stations. For scenarios with electrically neutral long-lived particles with $m\gtrsim 1$~GeV produced at the electroweak scale and above, the lifetime reach is increased by 2-3 orders of magnitude compared to currently operating and approved future experiments at the LHC. The original ANUBIS detector design proposal is outlined along with the projected costs.