Early July 2008 China mobilized more than 10,000 people and 1,400 boats into the fight to clean up a vast algal bloom threatening the upcoming Olympic sailing events in the sea area of east China's Qingdao, just one month before the start of the games (see Mar. Poll. Bull. 56, 1388). According to Xinhua news agency, the algae had arrived in late May and covered 13000 km2 of sea, occupying about a third of the area that was planned to be used for the sailing competition. By July 16, 1 million tonnes of algae had been cleared and Chinese authorities declared the Olympic sailing venue to be algae-free. Authorities and some scientists regarded the Qingdao algae bloom as a harmless natural phenomenon related to seawater temperature and salinity, wind and currents. Other scientists blamed the bloom on increased rainfall and warmer waters in the Yellow Sea and Olympic Sailing Committee officials believed that the harmful algae were a ''foreign enemytextquotedblright, having invaded the Qingdao coastal waters from an unspecified area offshore. A more detailed study carried out at the Universities of Ghent, Algarve and Belfast have shown that, morphologically, the alga unmistakably belongs to the green macro-algal genus Ulva. Species level identification in the genus, however, is confounded due to the lack of distinguishing characters and a high degree of morphological variability. Using phylogenetic analysis it has been shown that the bloom forming Ulva from Qingdao is very similar to U. procera and U. linza, with the Chinese Ulva genetic sequence being identical to those of specimens collected from Finland, Portugal, New Zealand and Japan. The Qingdao bloom is a typical illustration of a green tide, which is a type of algal bloom caused by proliferation of unattached green seaweeds, generally members of the genus Ulva. These blooms are triggered by a combination of different factors such as eutrophication, high temperature and irradiance level, and slow turnover of marine waters. The data reported here should help to eliminate some of the misconceptions surrounding its occurrence. U. linza has been reported to bloom in this area before and is hence certainly not a recently introduced ''foreign enemytextquotedblright. The green tide was likely caused by Ulva populations which were present in or near the coastal waters of Qingdao, and probably mainly triggered by a combination of eutrophication and high sea water temperatures.