The Secret Lagoons natural hot springs is located in the small village Fludir - in the Golden Circle area.
The water stays at 38-40 Celsius all year around and is surrounded by natural steam and a beautiful nature. During the winter, the northern lights often give a great lightshow above the lagoon. In the whole area there are several geothermal spots and a little Geysir which erupts every 5 minutes. Read more about the Secret Lagoon here: http://secretlagoon.is/