On Thursday there were reports of a mysterious loud explosion in the Rift Valley area of Taidong. I didn't actually hear it, but several years ago i heard sounds emanating from the mountains that fit the same description. I finally put it down to controlled explosions associated with a major roadworks project such as building a tunnel - but still don't know what it really was.

Other possibilities could be a secret military base, or possibly a natural gas escaping and exploding. It definitely wasn't thunder though.

A quick google search reveals something similar near Los Angeles - also unexplained.