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Steve Leonard is in Germany trying to find out how seals hunt fish in the dark. For this he has come to see a friendly harbour seal called Henry, a scientist called Guido Denhardt, and a remote-controlled submarine. Guido gets Henry to track down the mini-sub when it is hidden behind a gate. Henry does it in a second as it is daylight and the water is crystal clear. To simulate conditions in the deep sea, Guido has to make the game a bit more challenging. First, he gets Henry used to swimming with his eyes and ears blocked, then he runs the test again. Incredibly, even though unable to see or hear, Henry follows the sub’s exact path. It is therefore likely that seals would trail a fish the same way. But what sort of signal could both a submarine and a fish give out that could guide a seal in the dark? It is not scent as the sub has no smell. For more clues they turn to another team member, Wolfe Hanke, who studies fish swimming patterns. To show up currents and disturbance in the water, Wolfe uses sparkly particles and a laser to light it all up. Laser particle velocimetry reveals that fish still churn up the water as they swim, even though they are streamlined. Small fish leave a tell-tale wake that lasts at least 3 minutes. Larger fish, like the Antarctic tooth fish, leave wakes that last for more than 5 minutes - ample time for a seal to home in and make pursuit. It is the same with the sub but how does Henry pick up the trail? With no sight, sound or smell, there is just one option left - he must feel his way. Seals’ whiskers have 10 times more nerve cells than a cat’s. They can sense even the slightest turbulence in the water. A thermal camera shows how hot the whisker area is, pumped up with blood to feed thousands of nerve cells.