Ruki River in the Congo Basin (IMAGE) ETH Zurich Caption The water of the Ruki River in the Congo Basin is as dark as tea due to a high concentration of dissolved organic substances. Credit Photograph: Matti Barthel / ETH Zurich Usage Restrictions The rights to the images always remain with the person referred to as the copyright holder for them. Images published on ETH News may be used by other media and online portals only if ETH Zurich is specifically mentioned as the copyright holder and only in connection with the relevant ETH News article. Further distribution of images – for example, via download links – is permitted only with the approval of the editorial team. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.