Sigurlaug Dagsdottir, a graduate student researching the pools, speculated that the sundlaugs’ social utility in Icelandic communities derives in part from the intimacy of the physical experience: In the pool, she said, you can “take off the five layers of clothing that usually separate you from everyone else.” As such, the pools are a great leveler: Council members in Reykjavik make a point to circulate among the city’s sundlaugs, where they often take good-­natured grief from their constituents. The filmmaker Jon Karl Helgason, who is shooting a documentary about Iceland’s pools, said, “When people are in the swimming pool, it doesn’t matter if you are a doctor or a taxi driver.” His girlfriend, Fridgerdur Gudmundsdottir, added, “Everyone is dressed the same.”

On the way from Reykjavik to Keflavik airport is the Blue Lagoon, a luxurious hot-­water spa that is one of Iceland’s most popular tourist destinations. There, for 40 euros, you can shower in private stalls and float in mineral-­rich water — discharge from the nearby Svartsengi power plant, which uses turbines twice as tall as a man to generate 75 megawatts of electricity and 150 thermal megawatts of heat for the surrounding towns.

My final day in Iceland, I turned off the highway just after the Blue Lagoon and instead drove into one of those towns, the port Rekjanesbaer. The lobby of the town’s pool is dotted, fittingly, by a series of porthole-­like windows. The woman working at the desk charged me nine bucks and asked, “Is this your first time in an Iceland swimming pool?”

“Nope,” I said with some pleasure.

The familiar signs in the showers were supplemented by notices in Polish, targeting the new wave of immigrants who have found work in Rekjanesbaer. I snapped on my Speedo, steeled my courage and exited the warm lodge into the chill. The 36-­to-­38-­degrees-­Celsius hot pot was full of enormous men with Bluto-­type physiques and also a small girl in a pink ruffled bathing suit. The largest of the Blutos rose from the water, picked up the girl and carried her, giggling, to the family pool. His biceps sported a tattoo of a roaring bear consumed by flames.

This time I didn’t approach anyone, didn’t ask any questions. I didn’t speak at all. I concentrated on what I could feel: the water pressing lightly on my skin, the wind prickling my beard. All around me was the soft white noise of a community. The conversation; the connection; the freedom, within that flurry of sociability, to withdraw and simply be within yourself. It called to mind something a Ph.D. student named Katrin Gudmundsdottir told me on my first day in Iceland. She was describing a certain ineffable emotional state to me, a native Icelander’s sense of comfort while immersed in her neighborhood sundlaug. When I thought of what she said, a perfect G chord strummed inside me. “It’s not exactly like you’re happy,” she had mused. “It’s that you know how to be in the swimming pool.”

The sun was low on the horizon, bright but evanescent. The only other thing in the crystal-­blue sky was the contrail of a jet, pointed to the west. I closed my eyes. I was in the pool.