Blue stretched down as far as could be seen, illuminated by shifting shafts of down-welling light. Gazing down through this drop in the ocean I’d expected to see something, but nothing moved across the void, nor had it for hours. A container ship, like a skyscraper on its side, churned past us a few miles away. Then from the deep a shape glanced across the corner of my eye, there and not there at the same time. Coming back into view, a sleek, torpedo shape with a sinuous movement, well camouflaged against the inky backdrop: it was a large blue shark.