While driving south out of Owen Sound on highway 10, late in morning on dark cloudy day happened to look up and saw several black objects just below base of cumulus cloud. I could only look for a second or two then had to look at traffic. At first thought there must be birds flying just below the cloud. On second brief look now saw a semi-circle of black objects just below cloud base. Then had to look at traffic. On third peek up all was gone and just the cloud now normal in appearance. Later wondering about what I had seen it seemed the only explanation was a circular craft hovering just within the cumulus cloud and the base of the cloud had dispersed a little to just expose a bit of the circular bottom of a UFO. It all occurred in seconds while I took a second or two glimpse at what was happening above my ordinary line of sight while driving. That was my experience.