In the 12th–15th centuries, the Novgorodian Republic expanded east and northeast. The Novgorodians explored the areas around Lake Onega, along the Northern Dvina, and coastlines of the White Sea. At the beginning of the 14th century, the Novgorodians explored the Arctic Ocean, the Barents Sea, the Kara Sea, and the West-Siberian river Ob. The Ugrian tribes that inhabited the Northern Urals had to pay tribute to Novgorod the Great. The lands to the north of the city, rich with furs, sea fauna, salt, etc., were of great economic importance to the Novgorodians, who fought a protracted series of wars with Moscow beginning in the late 14th century in order to keep these lands. Losing them meant economic and cultural decline for the city and its inhabitants. Indeed, the ultimate failure of the Novgorodians to win these wars led to the downfall of the Republic.