 The Queen 's Gate Tunnel ( Welsh : <unk> <unk> y <unk> ) , also known as the Butetown Tunnel , is underneath southern Butetown and follows a line underneath the Wales Millennium Centre . The 715 m ( 2 @,@ 346 ft ) twin tunnel was constructed using the cut and cover method of construction , which involves digging a trench for the tunnel and then roofing it over . The tunnel was constructed using reinforced concrete and a central wall separates the two sets of traffic . 
