Work on the south shore of the harbour was the most complex and demanding. Beneath the Opera House forecourt, the land-marine transition structure was only accessible by tunnelling, and was constructed entirely underground. A large Moreton Bay fig, its roots just over the tunnel system, had to be preserved at all cost. As excavation was proceeding, the tree root system was propped up from within the tunnel, at a cost of $500k. "It was a bit like watching grass grow", quipped an engineer.
Tunnelling on the northern side of the harbour and excavations for the two ventilation stations presented similarly challenging problems, in particular digging under the sea level and connecting and driving the ventilation systems through and up the two northern pylons of the bridge.
Video: Sydney Harbour Tunnel.