A man walks on a beach covered by a layer of an unknown black substance in Qingdao city, East China`s Shandong province, Sept 4, 2011. The black layer forced a lot of people to stop swimming and leave the water, fearing the substance was oil from Bohai Bay, an area where an oil leak occurred at an oilfield jointly owned by ConocoPhillips China and China National Offshore Oil Corp. A sample of the black substance was taken to be tested by relevant departments.[Photo/CFP]

An unknown black substance is seen on a beach in Qingdao city, East China`s Shandong province, Sept 4, 2011.[Photo/CFP]

An unknown black substance is seen on a beach in Qingdao city, East China`s Shandong province, Sept 4, 2011.[Photo/CFP]

A piece of black material is picked up from a beach in Qingdao city, East China`s Shandong province, Sept 4, 2011.[Photo/CFP]