evidence: The Democratic Republic of the Congo (DRC) is the largest country of sub-Saharan Africa, occupying some 2,344,858 square kilometres (905,355 sq mi). Most of the country lies within the vast hollow of the Congo River basin. The vast, low-lying central area is a plateau-shaped basin sloping toward the west, covered by tropical rainforest and criss-crossed by rivers. The forest center is surrounded by mountainous terraces in the west, plateaus merging into savannas in the south and southwest. Dense grasslands extend beyond the Congo River in the north. High mountains of the Ruwenzori Range are found on the eastern borders with Rwanda and Uganda. The Congo River, formerly also known as the Zaire River, is the second longest river in Africa, shorter only than the Nile, as well as the second largest river in the world by discharge volume, following only the Amazon. It is also the world's deepest recorded river, with measured depths around 219.5 m (720 ft). The Congo-Lualaba-Chambeshi River system has an overall length of 4,700 km (2,920 mi), which makes it the world's ninth-longest river. The Chambeshi is a tributary of the Lualaba River, and Lualaba is the name of the Congo River upstream of Boyoma Falls, extending for 1,800 km (1,120 mi).