But this wasn’t tourism. Climate scientists predict that if warming trends continue, the Arctic’s ice cap will melt enough that — in this century — it will become a navigable ocean for commercial shipping year round, and for mineral and oil exploration. Russia has already made extensive claims to the Arctic, based on the reach of its continental shelf, beyond the usual 12 miles from its coastline; these are in dispute. To prepare for whatever unfolds here, though, the U.S. Navy keeps honing its Arctic submarine skills, including, on our trip, test-firing a virtual torpedo at a virtual enemy sub, studying how differences in water temperatures and the mix of freshwater from melted ice and saltwater affect undersea weapons and the sounds a sub makes (vital for knowing how to hide), as well as mapping the Arctic’s seabed topography.

“In our lifetime, what was [in effect] land and prohibitive to navigate or explore, is becoming an ocean, and we’d better understand it,” noted Admiral Greenert. “We need to be sure that our sensors, weapons and people are proficient in this part of the world,” so that we can “own the undersea domain and get anywhere there.” Because if the Arctic does open up for shipping, it offers a much shorter route from the Atlantic to the Pacific than through the Panama Canal, saving huge amounts of time and fuel.

You learn a lot on a trip like this, starting with the fact that I’m not claustrophobic. Sleeping in the middle rack of three stacked beds, appropriately called coffins, I now know that.

More important, you learn how crucial acoustics are when operating deep under ice with no vision and no GPS satellite to guide you. Or, as the New Mexico’s captain, Todd Moore, 40, put it: It’s like every day “engaging in a knife fight in a dark room: the only thing you can do is go after what you hear.” You can’t see the adversary. You can’t see the ice keels, but you can hear enemy subs, surface ships, whales, calving icebergs, schools of fish and bounce sound waves off them with sonar to measure distances. The New Mexico not only carries supersensitive sonar but also tows a giant electronic ear 1,000 feet behind it that can listen to the ocean without interference from the sub’s own engine noise.