Developed ostensibly as a structural integrity monitoring tool, the Structural Echo Tap transmits data by inducing precisely calibrated micro-vibrations into load-bearing concrete, rebar, and steel superstructures — the bones of GLMZ's perpetually under-construction cityscape. Receiving units detect and decode these vibrations using contact piezoelectric sensors pressed against the same structural members, reconstructing the signal from what appears to any passive observer as ordinary building settlement noise. The system was designed for Palladian Construction's own site supervisors to maintain covert communication channels in environments too electromagnetically hostile for conventional radio, but units have since proliferated well beyond authorized hands.