Last Thursday I attended the Canberra Engineering Wonders Forum, as part of science week. This will be on ABC Radio National "By Design" in a few weeks.
The panel was asked to nominate inspiring bridges and tunnels. They could think of lots of bridges but no tunnels.
The Sydney Anzac bridge must be one of the memorable bridges. Have a look at it from the air in fog.
At question time I asked about building a tunnel under the Himalayas to connect China to India, via Tibet. The tunnel expert said this was feasible. In "High Hopes As Tibet Railway Opens" (August 2006 International Railway Journal) Keith Barrow points out that there will only be a 100 km gap between the Indian and Chinese rail systems.
In terms of existing tunnels, the Channel Tunnel must be inspiring, for linking nations. Unfortunately it has not been a financial success. Also you do not actually see the tunnel, either the entrance portal nor tunnel walls, when travelling though it by train.