Small, spinning disc-shaped object seen by military helicopter over Seoul, South KoreaI am UH-60L Blackhawk crew chief with the US Army, I have over 350 hours flight time. We were doing a training mission through Seoul, and I was sitting right seat, behind the pilots. The time was 1901, about 20 minutes before official sunset, and the sky was hazy. We were flying North-West over the Anyangcheon canal at about 1000 ft, doing 100 knots. As we passed over the Ewha Women's University Medical Center, we flew right past the object.It appeared to be stationary, but very close to our flight level. It was approximately 30 feet off to the right side and 20 feet below us, at its closest point. It was spinning very fast. It looked disc-shaped, with a diameter that was no more than 2 feet. It was silver and reflective on top, and black underneath.I only saw it for 5 seconds before my pilots initiated a left turn and it passed out of my view. I informed my pilot-in-command of what I had seen and he said it was probably "something caught in a thermal updraft". I've never seen anything like that while flying.