The Northern Lights in a Geothermal Pool

The northern lights (aurora borealis) are caused by charged particles from the sun interacting with gases in the earth's atmosphere, producing green, pink, violet, and occasionally red curtains of light that move across the sky in patterns no algorithm has yet predicted. This explanation is accurate and entirely fails to convey what it feels like to watch them from an open-air geothermal pool at midnight in January with the temperature at minus ten above the water line and plus thirty-eight below it.

Reykjavik's most accessible version of this experience is the Nautholsvik geothermal beach on the south coast — a small sandy beach with a heated lagoon, free to use for a nominal fee, with clear views north across the Faxafloi bay toward the dark sky over the Snaefellsnes peninsula. The Secret Lagoon near Fludir and the Krauma Geothermal Baths near Reykholt are better options for a combined hot-spring and aurora experience outside the city.

The aurora is unpredictable — you can spend a week in Iceland in winter without seeing it, or see it on your first night. The conditions require: darkness (meaning at least a week from the full moon), clear skies, and solar activity above KP3 on the Kp-index scale. The Icelandic Met Office (vedur.is) publishes daily aurora forecasts. September, January, February, and March are the best months. Whatever else you do in Iceland — the Golden Circle, the waterfalls, the black sand beaches — the aurora is the experience that keeps people returning.