I am not kidding, he/she passed right underneath us about midway between Woorim and Tangalooma, a blue low wing, with nose wheel undercarriage, similar to a Sportstar or Sling, heading for Woorim. Not sure whether it was RAA or GA, as you can understand it all happened rather quickly. I was sitting on 3400ft as the C class starts at 3500ft, so he/she was doing some pretty dangerous stuff by heading that way. I may be wrong, and I know that under 5000ft he/she didn't have to, but I don't even think he was keeping to a hemispheric level, that is, I think he/she was a bit higher than 2500ft. I remember that I had just made my 10nm call to Tangalooma traffic when it happened and I was monitoring that frequency. There was a bit of traffic in the area that I could hear, but not him/her.There's always one in every industry, isn't there?