Which African river's mouth did the Portuguese name 'mighty' — the Zaire?

The Congo, and the name Zaire is a Portuguese rendering of a Kikongo word for river - so the country was for decades named after a European mishearing of the word river.

It is the deepest river in the world, over 220 metres in places, and second only to the Amazon by discharge. Its mouth pushes fresh water so far into the Atlantic that the plume is visible from space, and it has carved a submarine canyon extending 800 kilometres out across the sea floor.

That depth has produced something strange. The lower Congo has rapids and depths so violent that populations of fish on either side are effectively separated - genetically distinct species living a few hundred metres apart, divided by water too turbulent to cross. Some are blind and depigmented, adapted to deep dark water in a river rather than a cave. The basin also holds the second largest rainforest on Earth, and its peatlands, mapped only in 2017, store an amount of carbon comparable to several years of global emissions.