Almost 200 square miles (500 square kilometers) of the Yellow Sea off China are covered by a massive bloom of green algae, according to a report from China's Xinhua news service.
The bloom has spread to almost 7,400 square miles (19,050 square kilometers) in total and is expected to grow, Xinhua reported, citing the North China Sea Marine Forecasting Center of State Oceanic Administration.
The algae bloom threatens marine life as it sucks oxygen from the water although the algae itself is not poisonous, according to the Xinhua report.
Researchers don't know what causes the massive algae blooms, first seen in the Yellow Sea in 2007.
Tons of the green algae had to be removed in 2008 to make way for sailing events at the Summer Olympics.
"We don't know where it originated and why it's suddenly growing so rapidly," Bao Xianwen from the Qingdao-based Ocean University of China, told the China Daily earlier this month. "It must have something to do with the change in the environment, but we are not scientifically sure of the reasons."