Depth Runners are organized teams of urban explorers mapping and cataloguing the sealed sub-infrastructure of Old Chicago — flooded transit tunnels, pre-Consolidation bunker networks, the ruins of neighborhoods demolished to build the Shelf's lowest foundations. The Circuit is their informal competitive circuit: teams race to document and geo-tag new locations first, uploading encrypted proof-of-presence files to a shared Underworld repository called the Cartography of What Was. The most celebrated run in Circuit history was the 2221 breach of Sealed Zone Theta-9, a flooded district office complex containing intact administrative records from 2089-2103.