From Wikipedia:
Native to the northwest coast of the United States and Canada (primarily Washington and British Columbia), these marine bivalve mollusks are the largest burrowing clams in the world, weighing in at an average of one to three pounds (0.5–1.5 kg) at maturity, but specimens weighing over 15 pounds (7.5 kg) and as much as 2 meters (over 6.5 ft) in length are not unheard of.