The sheer feat of building a massive rounded structure for people to walk under is impressive, and these examples of tunnel architecture show just how much designers have pushed engineering to make distinct designs that work.
The Great Tunnel of China is one of the longest designs to come around in a while. The proposed construction will connect Dalian and Yantai, and run underneath the Bohai Sea. In total the tunnel runs 123 kilometers long and is meant for a train to run on the tracks. I personally would be slightly nervous and claustrophobic being underground for that length of time, especially underneath a sea, but for an efficient way to travel from one city to the next, this is definitely the best option.
Tunnel architecture is constantly pushing the bounds of what can and can't be done when it comes to building design.
12 Examples of Tunnel Architecture
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Trippy Tunneled Interiors
Titanic Tidal Tunnels
Twisted Design Tunnels
Trefoil Tunnel Monuments