Back in the day when we lived on Puget Sound, geoducks were there, but not much harvested because they lurked 2-3 feet down in soupy sand and were very hard to dig up due to the sand collapsing as you dug. You wound up squirming your entire arm down into the sand, with your cheek against the wet sand, and you had to get your hand under the critter to lift it. If you pull on the siphon, it just breaks off. We just went after the much easier clams, oysters, and sometimes spiny urchins. Those were good times!
They were favored for chowder, since they're pretty tough. As you probably know, the siphon/body is WAY too big to fit into their shell, so shucking them is no big challenge.
They're commercially harvested now, as I understand it, by divers with high-pressure hoses to blow away the sand and expose the beds so they can easily be picked up.