Blue Lagoon or Sky Lagoon: which one, and when to go

Both are geothermal seawater lagoons at about 38 to 40°C with swim-up bars, saunas and a spa building; the differences are the setting, the crowd and the logistics.

It is the one everyone has seen, and for good reason.

• It is your first visit and you want the milky-blue lava lagoon in the photograph; nothing else looks like it.
• It fits the flight: a morning arrival or an evening departure makes it the natural first or last stop, bags in lockers.
• You are traveling with children under 14, who go free (not admitted at the Sky Lagoon).
• You want the full spa day: the Retreat spa and the Lava restaurant are a cut above.

It is the one Icelanders recommend to friends.

• You are based in Reykjavík and would rather not spend two hours on a bus for a soak.
• You want quiet: adults only, no tour groups, the infinity edge at sunset.
• You like the ritual: the cold plunge and the sea-view sauna are the best part, and it is included in every pass.
• The Blue Lagoon is closed for volcanic activity, or sold out on your date.

With four days or more, do the Blue Lagoon on the airport side of the trip and the Sky Lagoon on a Reykjavík evening. In winter, time either for the sunset (around 16:00 in December) and stay in the water after dark; in summer, book the late slots, when the light is long and the coaches have gone.

The lagoons are the show; the pools are the habit.

• The town swimming pools: every town has one, geothermally heated, with hot tubs at 38 to 42°C, for $8 to $12. Laugardalslaug and Sundhöllin in Reykjavík, Hofsós on the north coast, and the pool in every village on the route.
• The Secret Lagoon at Flúðir on the Golden Circle: a natural pool from 1891, about $30, no ritual, all the charm.
• Hvammsvík on Hvalfjörður: eight pools at the sea’s edge an hour from Reykjavík, about $60, quieter than either lagoon.
• Krauma at Deildartunguhver, the Forest Lagoon at Akureyri, the Mývatn Nature Baths and the Vök Baths at Egilsstaðir: each region’s own, $40 to $60, worth a stop on the Ring Road.
• Free: Reykjadalur hot river (an hour’s walk from Hveragerði), Seljavallalaug in the South, Landbrotalaug on Snæfellsnes. Leave them as you found them.

What first-timers get wrong.

• Shower naked before entering, at both; it is the rule everywhere in Iceland and attendants enforce it.
• Condition your hair before the Blue Lagoon and keep it out of the water; the silica makes it straw for days.
• Book a time slot in advance at both; walk-ins are refused when a slot is full, and the afternoon slots in summer sell out days ahead.
• Two hours is enough in the water; the lagoons are warm but the wind is not.
• Both have lockers, towel hire, swimsuit hire, cafés and bars; neither needs anything from you except a slot and a swimsuit.