Martinslaug (Marteinslaug) is a geothermal lake in the Haukadalsskógur forest near the Haukadalur geothermal area.
This area is interesting because it is forested, which is very unusual in Iceland, but if you are after the experience of swimming in a geothermal lake, there are many other nice geothermal lakes in Iceland. Martinslaug is not really one of the nice ones. Not yet, anyway.