50km (31 miles) SW of Reykjavík.

The Blue Lagoon (tel. 420-8800; www.bluelagoon.is; June-Aug daily 7:30am-9pm; Sept-May daily 10am-8pm) is the most popular tourist attraction in Iceland, garnering around 360,000 visitors per year -- 20% more than Iceland's entire population. The surreal image of bathers in milky blue water, faces smeared with white mud, backgrounded by a power plant amid an expanse of black lava, may be the most common photographic emblem of an Iceland vacation. Not everyone sees what the fuss is about, but all are guaranteed to walk out with that fresh mineral tingle.

What is the Blue Lagoon -- When the neighboring geothermal power plant was built, deep boreholes were drilled to extract pressurized, blazing-hot, mineral-rich water from thousands of feet underground. This water produces electricity by driving steam turbines. The runoff water, still very hot, is too salty to provide central heating for homes, so it was piped into the lagoon, which was dug out of the lava field. The water gets its pearly, bluish color from the combination of algae, silica, and other minerals. (In high summer, when the algae is in full bloom, the water is more green than blue.) One day, an Icelander suffering from psoriasis popped in for a swim. He noticed an immediate improvement in his skin condition, and the rest is a textbook marketing success story. (The "Blue Lagoon" was named not long after the Brooke Shields movie.) Recently a bank of lava was added, to shelter swimmers from the wind and to block views of the power plant. No doubt this was intentional, but some of the sci-fi ambience is lost.

The spa has drilled its own water boreholes to regulate the temperature and balance the mineral and salt content. The salinity matches ocean water and reduces the eggy smell normally associated with geothermal water. Other spas have mimicked the Blue Lagoon's success, but none can quite duplicate its water formula. Studies have apparently confirmed the water's effectiveness for psoriasis, and the public health system even covers some visits here. Claims made for curing arthritis, baldness, negative karma, and the like are less reliable. The two most common ailments treated by the Blue Lagoon are jetlag and hangovers.

The Lagoon Experience -- No orientation lecture here -- basically you're just handed a locker key and sent off to shower and jump in a hot lake. (Make sure to remove your jewelry; the water damages precious metals, especially silver.) White silica mud, which conditions and exfoliates the skin, is scraped from the bottom and left in buckets for guests to smear all over themselves. The water is around 100°F (36°-39°C), but the temperature can change abruptly as you move around. Watch out for the rectangular wood structures, where hot water is introduced. One billows steam just for effect. The water is completely replaced by natural flow every 40 hours.

Minerals in the lagoon water will coarsen and harden your hair. No real harm is done, but conditioner -- provided free in the showers -- should be left in while you soak, and long hair should be tied up. If growing dreadlocks, reverse these instructions.

It's a very mixed scene: Icelanders are interspersed among visitors -- some of whom aren't sure they want to share a bath with so many strangers. (Be reassured: the water is completely replaced by natural flow every 40 hours.) Private locker rooms will soon be added for those who don't like showering en masse.

Any time of year is great for a visit. Winter nights may be best of all, since you can watch the northern lights in complete comfort with icicles forming in your hair.

Basic admission (40€ off-season, 50€ summer; 25€ children 14-15; 13 and under free) includes entrance to the lagoon, sauna rooms, and a pummeling waterfall. BYOT, otherwise towel rental is 5€, and suit rental is 5€.

Once you've had an in-water massage, you might never go back to on-land ones. You lie floating on your back in womblike weightlessness, with a blanket over you, while the masseuse's hands slide between your back and the floating mat. Call or check the website for a full menu of fabulous spa treatments. Book well in advance.