The IMT structure consists of eight precast concrete units. The units were constructed over away in a casting basin at Port Kembla and then towed to Sydney Harbour. A trench was dredged before the arrival of the IMTs and then the IMTs were lowered into the trench by a system of pontoons and control towers. After the IMTs were in place the trenches were backfilled and then a rock armour was placed over the top to protect the units against marine hazards, such as anchors or sinking vessels. The land tunnels were constructed by a combination of driving and cut-and-cover techniques, designed to be strong enough to withstand the impact of earthquakes.