<p>A reliable source in Seoul's diplomatic community says Sunday a mushroom cloud with a radius of 3.5 to 4 kilometers was spotted in Kimhyongjik County in North Korea's northernmost inland province of Yanggang on Sept. 9.</p>
<p>SEOUL, South Korea - A large explosion rocked the northern part of North Korea (news - web sites), sending a huge mushroom cloud into the air on an important anniversary of the communist regime, South Korea (news - web sites)'s Yonhap news agency reported Sunday.</p>