In the late 17th century, two traders built a trading post on the Roanoke River in what is now downtown Plymouth, and by 1738 the site was called Brickhouse Plantation on maps. A shipyard was built there in 1771, a settlement known as Plymouth Landing developed, and cedar shingles, lumber, turpentine, and other freight were shipped from it. In 1790, a group of investors bought Plymouth Landing, and later that year the U.S. Congress designated the settlement as a port of delivery. The Dismal Swamp Canal, dug by enslaved laborers through 22 miles of swamp, linked the Albemarle Sound in North Carolina with the deepwater harbor in Norfolk, Virginia, when it opened in 1805, giving northeastern North Carolina a link to larger cities in the northern U.S. and abroad. In 1815, William J. Lewis and other explorers sailed a small flat-bottomed boat from the headwaters of the Roanoke River in the Virginia mountains to the Albemarle Sound and then through the canal to Norfolk, showing that goods could be sent from hundreds of miles upriver to Norfolk's harbor; an account of the expedition was published in Niles' Weekly Register in July 1816. In 1805 there were 16 states in the United States of America, and slavery was legal in 11 southern states, including North Carolina, where African Americans could be owned as property and forced to work without pay. Enslaved people worked on plantations, at commercial fisheries, and in other businesses, and some left the places where they were held and tried to reach northern states where slavery was illegal, or Canada, by traveling along secret routes and staying in safe houses run by sympathetic people; this network became known as the Underground Railroad, and it ran through Plymouth. In the 19th century, the Plymouth waterfront was a busy seaport where freedom seekers could enter what author David Cecelski called a maritime underworld, mingling secretly with ships' passengers, free and enslaved dock workers and sailors, and ships' crews. Enslaved men loaded and unloaded ships on the docks, and enslaved people also worked as cooks, cleaners, carpenters, pilots, sail makers, riggers, caulkers, and dock workers. Some people in the busy crowd on the docks were willing to help freedom seekers. Underground Railroad activity also took place in the swamps and forests near Plymouth. In the late 18th and early 19th centuries, the wilderness around the Albemarle Sound was the center of the naval stores industry, which produced turpentine and other products used to build and maintain wooden sailing ships. Turpentine work camps deep in the wilderness, far from law enforcement and slave patrols, were ideal places for freedom seekers who needed money because workers were hired on the spot with no questions asked, paid in cash, and able to leave on a moment's notice to continue their journeys.