Many extensions of the Standard Model with Dark Matter candidates predict new long-lived
particles (LLPs). The LHC provides an unprecedented possibility to search for such LLP produced
at the electroweak scale and above. The ANUBIS concept foresees instrumenting the ceiling
and service shafts above the ATLAS experiment with tracking stations in order to search for
LLPs with decay lengths of O(10m) and above. A prototype detector, proANUBIS, has been
constructed within the ATLAS experimental cavern to demonstrate the ANUBIS concept and
verify the expected performance. The proANUBIS design and initial performance during 2024
LHC collisions are presented, and the planned analyses using prototype data are discussed.

