Tens of thousands of black headed gulls dive for food or bathe, offering a rare and spectacular view to tourists as a result of the astronomical tide on the coastal area near the Zhanqiao pier in Qingdao city, Shandong province, Dec 28, 2015.The phenomenon reveals a beach with the maximum size at the ebb tide hour. Astronomical tides happen when the combined effects of gravitational forces exerted by the sun and the moon reach their strongest, during which sea levels can rise to the highest at high tide and fall to the lowest at the ebb.[Photo/Xinhua]