How do you prepare for cold-water diving? Before we hit the road for Iceland, that’s the question we posed to National Geographic photographer Paul Nicklen. Paul’s a guy who could probably live comfortably in his freezer. He’s shot stories in Antarctica, the Canadian Arctic, and Svalbard, Norway, at the very top of the world. He has dived under ice and been hit by an airborne penguin. Early next year, you can see his National Geographic feature on the Yukon.

Spencer and I are both certified divers. But when it came to diving Iceland’s inland fissures—water temperature 2°C (35° F)—here’s what he told us:

We’ll have lots more to say later this week about our diving in Iceland—specifically why we did it,