Yakutsk soon turned into a major base for further Russian expeditions eastward, southward and northward. Maksim Perfilyev, who earlier had been one of the founders of Yeniseysk, founded Bratsky ostrog in 1631, and in 1638 he became the first Russian to enter Transbaikalia. In 1639 a group led by Ivan Moskvitin became the first Russian 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 proximity of the Amur River. In 1640 they apparently sailed south, explored the south-eastern shores of the Okhotsk Sea, maybe even reaching the mouth of the Amur River and discovering the Shantar Islands on their return voyage. Based on Moskvitin's account, Kurbat Ivanov draw the first Russian map of the Far East in 1642. He led a group of Cossacks himself in 1643 to the south of the Baikal Mountains and discovered Lake Baikal, visiting its Olkhon Island. Subsequently, Ivanov made the first chart and description of Baikal.