Conflict intensified near the disputed maritime boundary known as the Northern Limit Line in the Yellow Sea. In 1999 and 2002, there were clashes between the navies of North and South Korea, known as the First and Second battle of Yeonpyeong. On 26 March 2010, a South Korean naval vessel, the ROKS Cheonan, sank, near Baengnyeong Island in the Yellow Sea and a North Korean torpedo was blamed. On 23 November 2010, in response to a joint military exercise, North Korea fired artillery at South Korea's Greater Yeonpyeong island in the Yellow Sea, and South Korea returned fire.