of the tube. The doors are locked from the gallery side and can be opened inwards (toward the gallery) from the tunnel through emergency hardware. Between doors, the tunnel has narrow wide walkways adjacent to the gallery space. The upper section of the gallery space is also used as a duct, moving of air under forced circulation. The tunnels are vented to the atmosphere at the San Francisco and Oakland ends and are vented to each other (through the upper gallery) with remotely-operated dampers long by high over every third door. Each end of the tube is secured to the vent structures with a patented sliding seismic joint which allows six degrees of freedom (translation along and rotation about three axes). As-designed, the joints allow movement of up to along the tube's axis and up to vertically or laterally. A restaurant was constructed atop the San Francisco transition structure (vent) on a pier behind the Ferry Building. The Oakland vent structure is located in the middle of a Port of Oakland container yard. Seismic retrofitting The Transbay Tube has required earthquake retrofitting, both on its exterior and interior. The total cost of seismic retrofits was estimated at in 2004. A 1991 study, commissioned at the recommendation of the Governor's Board of Inquiry in the wake of the 1989 Loma Prieta earthquake, found the seismic joints would "likely remain intact and functional after the next earthquake." However, settling of the tube within its trench and the Loma Prieta quake had reduced the allowable movement of the seismic joints to as little as . The 1991 study was followed by a more detailed BART Seismic Vulnerability Study, published in 2002, which concluded the fill packed around the tube might be prone to soil liquefaction during an intense earthquake, which could allow the buoyant hollow tube to break loose from its anchorages or cause movement that would exceed the capacity of the sliding seismic joints. Retrofitting work required the fill to be compacted, to make it denser and less prone to liquefaction. Compaction started in Summer 2006 at the east end of the tube, on property belonging to the Port of Oakland. A 2010 paper concluded the distance the tube would rise due to liquefaction was limited based on model testing of potential liquefaction mechanisms, and questioned the justification for the compaction effort. On the interior of the tube, BART began a major