Interest was renewed again between 1819 and 1843. As Europe settled after a period of war and unrest, explorers Fabian Gottlieb von Bellingshausen, John Biscoe, John Balleny, Charles Wilkes, Jules Dumont d'Urville, and James Clark Ross sought greater knowledge of the Antarctic regions. The primary goal of these explorers was to penetrate the vast barriers of sea ice that hid Antarctica proper, beginning with Bellingshausen and Mikhail Lazarev's circumnavigation of the region in 1819–1821, during which they became the first to sight and therefore officially discover mainland Antarctica, and culminating in Wilkes' discovery of Victoria Land and naming of the volcanoes now known as Mount Terror and Mount Erebus in 1840. Much early knowledge of the lands south of the Antarctic Circle was also derived from economic pursuits by sealers and whalers, including the probable first landing on mainland Antarctica by an American sealer in 1821, though whether this landing was truly the first is disputed by historians. These explorers, despite their impressive contributions to South Polar exploration, were nonetheless unable to penetrate the interior of the continent, and their discoveries instead formed a broken line of newly discovered lands along the coastline of Antarctica.