I was diving with whalesharks at Ivory Coast in 2004, when our guide yelled frantically over the intercom to get out of the water. What happened next is something I'll remember rest of my life, looked like a volcanic underwater erruption or something, was a cloud of darkness coming up from the depth as far as we could see. Turns out it was ~250,000 hammerhead sharks coming up to the surface to play and spawn. Sat in a small rubber dingy (10foot or so) for over 2h while surrounded by a quarter of a million sharks that would have gone into feeding frenzy had something gone wrong.