The phrase "to build the tunnel" means constructing a pathway under the ground or water, typically using excavation equipment and machinery. Full definition
If you can't get over the mountain, then build a tunnel through it or a road around it. (remonline.com)
Officials are considering building a tunnel underneath the runway in order to improve the territory's limited infrastructure. (politics.co.uk)
Now I do not know much about building tunnels, but thanks to previous lives as an architect and real estate developer, I do know something about building housing. (treehugger.com)