A menacing fin pierced the surface and sliced toward us. A great blue shark—three meters in length—homed in on the scent of blood like a torpedo. As my wife, Melanie, and I watched several large sharks circle our seven-meter Boston Whaler, a silver-blue snout suddenly thrust through a square cutout in the boat deck. “Look out!” Melanie shouted. We both recoiled instinctively, but we were in no real danger. The shark flashed a jagged smile of ivory saw teeth and then slipped back into the sea.

We had drawn the sharks by ladling blood into the ocean, but we were not interested in their well-known attraction to blood. Rather we were investigating the hunters’ mysterious “sixth sense.” Laboratory research had demonstrated that sharks can sense extremely weak electric fields—such as those animal cells produce when in contact with seawater. But how they use that unique sense had yet to be proved. We were on that boat to find out.