For centuries, European nations and later the United States sought a Northwest Passage linking the Atlantic and Pacific oceans, and in that search they explored and claimed parts of the Pacific Northwest. According to Chinese texts, the Buddhist monk Hui Shen traveled in 499 to a land called Fusang about 10,000 kilometers from mainland China, which some suggest was the Pacific Coast of North America, though no other evidence remains. In the 1700s, Russians, Spanish, and British explored the Northwest Coast, and a Spanish shipwreck occurred off the Oregon Coast in 1707. In 1775, Spanish explorers landed at points on the Washington and Oregon coasts and claimed territory. During the 1770s, smallpox struck the Northwest Coast, and epidemics intermittently plagued the region for the next 75 years. In 1790, agreements between Spain and Great Britain began at the Nootka Convention, and both countries took license to settle in this un-colonized land. At the time of European contact in the 1790s, the Chinook population numbered in the tens of thousands from The Dalles to the Pacific Ocean. On May 11, 1792, American Captain Robert Gray reached the Columbia River and explored 20 miles upstream; in October 1792, British Captain George Vancouver arrived there, and Lieutenant William Broughton explored nearly a hundred miles upriver to the mouth of the Willamette River. In the 1790s, American and English traders regularly visited the mouth of the Columbia River, primarily to trade for furs. In 1803, France sold Louisiana to the United States, and President Jefferson sought to unite the nation coast to coast. In 1805, Meriwether Lewis and William Clark led the Corps of Discovery to the Pacific Ocean, the first American overland expedition to cross the continent, and on November 15, 1805, they established Station Camp here for ten days. Lewis and Clark counted 7,760 Chinook along the lower Columbia, two decades after disease had already reduced the population. From the 1790s to the 1820s, the coastal Chinook engaged with fur traders of the American Pacific Fur, and British North West and Hudson's Bay companies, which also relied on them for food, other goods, and labor. In the ensuing years, the maritime fur trade brought hundreds of merchant vessels to the Columbia River, and land-based traders soon went far into the interior of the Pacific Northwest. In 1811, John Jacob Astor established Fort Astoria for the Pacific Fur Company at the site of the modern town. In October 1813, because of the War of 1812, Astor sold the American-owned, Canadian-staffed Pacific Fur Company to the British Northwest Company, which renamed it Fort George. In 1818, the Treaty of Ghent allowed joint occupation of Oregon Country by Great Britain and the United States. The Northwest Company merged with the Hudson's Bay Company in 1821, and in 1825 they moved their operations to Fort Vancouver.