A stretch of China's Yangtze River has mysteriously turned red around the city of Chongqing. Officials are investigating the river's transformation, as nobody is quite sure what caused it. The river began turning the color of a nice marinara sauce on Thursday.

The Yangtze River is the longest in Asia and the third-longest in the world. The affected area of the river -- reportedly turned the color of tomato juice is generally in the vicinity of the industrial city of Chongqing, although red sections of the river have been reported elsewhere, as well.

While some citizens are concerned about the river turning red, others are interested by the transformation. The Daily Mail has several photographs of Chongqing inhabitants fishing in the water and filling bottles with the red river water to show off later.

Scientists are looking to a natural cause for the river's change in color. Emily Stanley, who researches limnology (the study of inland waters) at the University of Wisconsin, believes it is possible microorganisms could be behind the sudden change, but that it is probable there is a much better explanation for it.

When water turns red, the thing a lot of people think of first is red tide, Stanley told LiveScience. But the algae that causes red tide is a marine group and not a freshwater group, so it's highly, highly unlikely that this is a red-tide-related phenomenon.

She acknowledged certain other microorganisms can turn freshwater such as lakes red due to oxygen deficiencies. However, it is much more uncommon to see a moving body of water change color, as a river's constant motion interferes with this process.

Stanley also indicated an industrial pollutant could be a likely cause for the river turning red.

It looks like a pollutant phenomenon, she said. Water bodies that have turned red very fast in the past have happened because people have dumped dyes into them.

The Daily Mail noted that last December the Jian River was turned red after two illegal garment workshops dumped tons of red dye into the river. Chinese officials investigated the river's transformation and shut down the operation.

Finally, there is a possibility that silt from an upstream area of the river could have been carried down to Chongqing and colored the water.

China is well known for having areas with a lot of steep hillsides and a lot of land-use practices that promote soil erosion and soil going into rivers, Stanley said. You can get red-colored clays that wouldn't be a whole lot different from having a big dose of dye go in there. But if that's the cause, I'd imagine there would have had to be a huge storm or a huge amount of clay go into the system.