Some years ago, in the Dry Tortugas, I was cruising solo along a shelf which lies at about 80 feet, and then drops off sharply to about 150 feet. I freeshafted a Black Grouper of about 25 pounds who was being rather uncooperative about being put on my stringer. A Bull shark swam directly up over the edge and stopped, looking at me (or the Grouper) at a distance of about 8 feet. The bull was probably about 6 or 7 feet long, but head on at that distance looked VERY big. Now, I had the Grouper in one hand, the stringer in the other, my unloaded gun with another freeshaft was lying on the bottom a few feet away, my powerhead was in my wetsuit sleeve. I remember thinking "this could be bad". After a few seconds, which seemed like an hour and half, the shark turned and casually swam away. I has a lot of air left, and ordinarily, would have continued to hunt, but in this instance I headed for the boat. Over the years, I have had quite a few shark related episodes, but this one has always stood out in my memory.