The Underground, The Tube, The Metro
The ground below iron is iron under the ground. It isn't a deposit of iron, though. It's man-made. And what's made of iron? Trains. In this case, because it's underground, it's a subway. Depending on where you live, you might call it the Underground, the Tube, or the Metro
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
I went to college using a subway called "Tokyo Metro."
There are subway exits in various places.
There are many different kinds of people on the New York subway.