Many European maps of North America in the early seventeenth century depict a massive inland sea, hundreds of miles in diameter, with a small inlet and many interior islands. The origins of this myth can be traced to several different sources, and it would not be until the great Russian expeditions of the mid-eighteenth that knowledge to disprove the idea would be obtained. Like many cartographic myths, the Sea of the West persisted for decades after it was proved wrong.
The 1592 voyage by Juan de Fuca is celebrated as discovering the Salish Sea for Europeans. Of course, de Fuca's travels are little known, and it is uncertain if he ever even sailed into the straits which now bear his name. Navigators such as Cook doubted him, but it now seems that de Fuca's account of his voyage matches strongly with the geography of this area. De Fuca describes a large bay with numerous archipelagos which he spent many days sailing. Intriguingly for those seeking a Northwest Passage, he described a vast inland sea which he saw but did not reach.
This idea of an inland bay was picked up by De L'Isle, who drew several conjectural maps which included a Sea of the West, though these were never printed. A copyright battle prevented competitors from following up on this idea, though it had commenced gaining traction.
The myth of the Sea of the West was picked up again at some point in later in the eighteenth century following the supposed discovery a long-lost letter regarding the Spanish admiral Bartholomew de Fonte's travels to the region. It would be this iteration of Sea which would make its way onto later maps. The exact nature of this Sea varied from map to map, but the grandest ideas depicted a vast body of water stretching nearly to the Mississippi.
While not all maps of the period depicted such a sea, many did. Russian exploration to the northwest was the primary reason this short-lived myth was dispelled, even though initial voyages had not ruled out such a bay. Gerhard Muller's map is perhaps the most influential of the period to not show this bay, and his work was responsible for laying the cartographic foundations for a Pacific Northwest we now recognize today.