The most famous tunnel is the thirty mile “Chunnel” between England and France. Thirty miles of air is a lot to push around, so engineers have come up with a clever solution. The Chunnel is actually two separate tunnels, connected by cross passages.
Also, earth’s core is a metal sphere 800 miles wide, but just pretend we’ve bored through it. What would be the result? For one thing, you could then travel to the other side of the world in under one hour.