No one originally had to 'Dig' these tunnels, these were naturally formed during the creation of the island by the Volcano. They are called 'Lava Tubes', Real simple, someone found the Volcano lava tube system, and improved them.

There's an underground city in Everett Washington. I've been to the elevator in an old abandoned building, but I was by myself and too scared to go down alone. I'm still very curious.

Everett, like Seattle had massive fires in past history that destroyed parts of the city. They built new city over the top of the burned out city. In seattle you can take tours of the underground tunnels even.