Ruki, a tributary of the Congo River, has recently been labelled the darkest river in the world, with water so dark that you can’t even see your face in front of you.

In what is considered the first-ever scientific study of the African river, scientists concluded that the dark-coloured water is caused by the high levels of dissolved organic matter from the surrounding rainforest.

Photo Credits: Matti Barthel / ETH Zurich

Using a measurement system, the team of scientists from ETH Zurich, in Switzerland, found that Ruki was 1.5 times darker than the Amazon’s Rio Negro, the world’s blackwater river. Although the Ruki only makes up a twentieth of the Congo Basin, it provides a fifth of all dissolved carbon in the Congo.

The Ruki is one of the most DOC-rich river systems in the world

Matti Barthel, the study’s co-author, said. Its water contains four times as many organic carbon compounds as the Congo’s and 1.5 times as much as the Rio Negro’s in the Amazon.