I lived in Portland for 2 years and never knew about this! Seattle has an "underground", too, but its origin was different: the level of the streets in downtown Seattle was raised one story to prevent the constant tidewater flooding. This left the original street levels of buildings as an unused "underground". Now you can take an enjoyable tour of "Underground Seattle"; if "Underground Portland" isn't a full-fledged tourist deal now, I'm sure it soon will be.

And shouldn't that be Northwest 23rd St.? Or am I forgetting my Rose City geography?

Aloha,
Mark