Downtown Missoula's first deep infiltration gallery has been a significant learning experience. The three-pipe gallery, three 96-foot, 48-inch perforated CMPs preceded by a Contech HDS separator, was designed to accommodate roughly 440 gpm at a conservative 24 in/hr infiltration rate, later reduced to 12 in/hr at the city's request. Soon after startup, constant dry-weather flow at the outfall indicated the facility was not performing as intended. A non-recirculating HVAC system at the historic Florence Building was continuously discharging to the storm system, saturating the subsurface and consuming hydraulic capacity. A pre-construction water main break had also left sediment in upstream mains, which flushed into the gallery upon startup, depositing 2.5 to 3 inches of material along the floor. City staff, wastewater crews and the Fire Department executed a confined-space entry nearly 30 feet below grade to manually remove approximately 5.5 cubic yards of sediment. Post-cleanout observations suggest the facility is now functioning as intended.