While Iceland may be frigid on the outside, it's molten lava on the inside. The most exciting evidence of the heat within is provided by the island's many geysers.

Geysir, the original blowhole from which all others get their name, now lies dormant on a grassy slope below Bjarnfell Mountain, 120 km east of Reykjavik. But Geysir's neighbor, Strokkur, is positively explosive; thousands flock yearly to watch it fire off a dazzling 30-m spout of scalding water every few minutes.

Icelanders take full advantage of their country's volcanic potential: when the mercury drops, they warm up by plunging into 38-40C thermal waters.

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nginx/1.14.0 (Ubuntu) One of the steamiest spas is the Blue Lagoon, a 40-minute drive from Reykjavik, and set dramatically in a wilderness of moss-covered rocks.

On a cold day its milky blue waters resemble a misty version of Heaven. Spa options range from massages on a float in the water to a facial with the lagoon's silvery-gray silt, which is said to have healing properties. Inevitably, there's also a boutique selling the Blue Lagoon's very own cosmetics, and an excellent seafood restaurant. Warning: stray too close to the lagoon's source while having a soak, and you'll quickly learn just how hot Iceland is beneath its frosty surface.

• AATISH TASEER
• Go see the original old geysers for a blow-out to match them all