Fearing English and Dutch penetration into Siberia, Russia closed the Mangazeya seaway in 1619. Pomor activity in Northern Asia declined and most Arctic exploration in the 17th century was carried out by Siberian Cossacks, sailing from one river mouth to another in their Arctic-worthy "kochs". In 1648, the most famous of these expeditions, led by Fedot Alekseev and Semyon Dezhnev, sailed east from the mouth of the Kolyma River to the Pacific Ocean, and rounded the Chukchi Peninsula, thus proving that no land connection existed between Asia and North America.