The Congo River system drains an area the size of Europe fed by ten thousand streams. It stretches from Tanzania in the east, to Zambia and Angola in the south, Gabon and Cameroon in the west and the Central African Republic in the north. It takes six months for water to go from the source to the Atlantic 4,700km (2,900 miles) away.

Two rivers, the Luapula and the Lualaba, form the headstream of the mighty Congo River. The Lualaba flows northwards and becomes known as nZaire - literally 'the river that swallows all rivers' - also known as the Congo. The Luapula is a huge river that flows westwards into the belly of Central Africa. After turning an enormous bend to flow north, it's then joined by the outpouring from another great basin: Africa's deepest body of fresh water, lake Tanganyika.

Below Kisangani the ancient Congo River widens and calms. The river has flowed here since before the continents broke up, when Africa was still joined to South America. Scientists even speculate that once upon a time the Congo and the Amazon were two branches of the same colossal river. The lower river is an endless labyrinth of channels and streams. Fish in the middle reaches have evolved in isolation for 50 million years, cut off at either end by rapids and waterfalls. As a dozen broad tributaries join the flow, the Congo River grows so wide that for many land animals it creates an impassable barrier.

The Congo River is transformed from a lazy snake into a wild torrent as it's thrust down a series of narrow gorges and cataracts extending for about 354km (220 miles) between Kinshasa and Matadi. The total drop of the falls is about 260m (850ft). The falls, named after the Scottish explorer-missionary David Livingstone, were crossed in 1877 by Sir Henry Morton Stanley, who charted the course of the Congo River.

Two to three thousand years ago, the Congo jungle areas were largely grassland. Over the past few million years, Africa's climate has swung between extremes of high temperature and humidity and cooler, dryer periods, and in response, the forests have waxed and waned like a tide over the Congo basin.

Open patches in the midst of the forest - called bais - have been a major discovery here. Some are simple arenas of grass. Others, like Wali bai, contain streams or pools. The bais brim with life, as forest creatures home in on these oases of light in the midst of darkness. The bais attract large mammals out of the forest. Even the least known of the Congo primates, the western lowland gorilla.