On a clear, calm day, the water on Challenger Bank off Bermuda is so clear it looks like air. ⁠ ⁠ It is another atmosphere, separated from the one above it only by the thin silvery film of its surface. But if you look down into it, the water assumes substance and turns a solid, deep blue. You can only appreciate the clarity of the water if you are swimming below the surface or if some object appears in it to fill your eye. Something did appear, and it did so with unnerving suddenness. It happened almost three hours to the minute after I first hooked into a big yellowfin tuna. The tuna fought deep, and the tussle had been hard enough to strain my back and make my arms ache and quiver, but the fish was finally coming up. I wanted him very badly, mostly to prove that I could take him on the 18-pound-test line I was using, but also because I thought there was a wild chance he was big enough to break the 20-pound test line-class record. Then a massive world-record class tiger shark got invo...