Marking All Unread in Gmail

I accrue unread messages in my Gmail account. Press releases, conversations I've dealt with in other contexts. Whatever. They add up, and by now many of them are so old that I'm never going to deal with them. Which is why it's annoying that Gmail doesn't have an easy way to just mark everything as "read."

Except it does. On a whim, I tried searching for "label:unread" in Gmail and voilà! All my unread messages. I had to go through about three pages, select all, mark as read, and go to the next. But that took less than a minute. Sweet.