Puget Sound is the second largest estuary in the United States after Chesapeake Bay. A sound is an arm, inlet, or recessed portion of the sea. Advancing glaciers carved out the Puget Sound basin about 15,000 years ago. Kelp was traditionally used by Native Americans to make fishing line and hold liquids. Today, an extract called algin from kelp is used as a thickener in ice cream, lotion, and paint. More than one-third of Washington's 2,300+ miles of shoreline has been modified by human activity.