Tunnel, Mine, Shaft
noun
A pit that's used as a road is called a tunnel. Or a mine, if they're taking valuable things out of it.
こうどう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
地下20mのところに坑道が掘られています。
They’re digging a tunnel somewhere 20 miles underground.