Concerns over environmental problems spoiling Beijing’s Olympiad have usually centered on the city’s air quality, but a new threat to the Games has materialized in the sea. The waters off the coastal city of Qingdao, the venue for the Olympic sailing events, have become choked with thick, green algae.

The bloom snakes along the shore and covers a third of the Olympic course, according to the state-run Xinhua News Service – and the muck is making life difficult for sailors and windsurfers who have come to train ahead of their August events. For Qingdao, a former German concession best known as the home of Tsingtao Beer, the outbreak is a monumental headache just six weeks before the Games begin.

More than 10,000 troops and close to 100,000 volunteers have been deployed to battle the algae, says Gao Zhenhui, director of the State Oceanic Administration’s North China Sea Environmental Monitoring Center in Qingdao. “At first we didn’t realize how big it would be,” Gao says. “We didn’t think it would happen so fast.”

Last June, Qingdao saw an algae blooms that covered 27 square miles, and a second one in September covered three square miles. But those are dwarfed by the current algae bloom, which covers 154 square miles.