As many as 320 pieces of pottery were pillaged off the coast of Yami Island, Gunsan Jeonbuk, including 176 celadon soup bowls, 98 celadon dishes and 46 celadon bowls which are presumed to date back to the Goryeo Dynasty in the 12th Century. Seoul Namdaemun police issued arrest warrants Friday to detain two grave robbers.