In 1639 a group of explorers led by Ivan Moskvitin became the first Russians to reach the Pacific Ocean and to discover the Sea of Okhotsk, having built a winter camp on its shore at the Ulya River mouth. The Cossacks learned from the locals about the large Amur River far to the south. In 1640 they apparently sailed south, explored the south-eastern shores of the Okhotsk Sea, perhaps reaching the mouth of the Amur River and possibly discovering the Shantar Islands on their way back. Based on Moskvitin's account, Kurbat Ivanov drew the first Russian map of the Far East in 1642.