Complex sentence: It wasn't reported until September 12, when the South Korean news agency Yonhap cited a source in Beijing, China who said a mushroom cloud had been seen, and a resulting crater from the explosion was later imaged by satellite.
Split into simpler sentences: It wasn't reported until September 12, when the South Korean news agency Yonhap cited a source in Beijing, China who said a mushroom cloud had been seen. A resulting crater from the explosion was later imaged by satellite.