Something which is disgusting, grisly, or gruesome - but both in spite of and because of that, those who gaze upon it find themselves unable to look away for a long period of time, and may even find themselves watching it again and again, if possible.
There was always something morbidly hypnotic about the way my father would use a knife to scrape the black stuff from under his toenail, wipe it on his knee, and repeat, until all nine toes were scraped clean.