Passing through the Keflavík airport perhaps 10 times over the years (it's the shortest flight between Oslo and Seattle), I've seen the ever-present ads for the Blue Lagoon. The way it's marketed, it just seemed to me like something touristy to avoid (e.g. crowds, hype, disappointment).
Perhaps because it's December, cold, snowy and dark, the experience was much better than I expected. After a required nude shower, you don your swimsuit, exit the building and walk some 15 meters in the freezing wind before entering the lagoon. The water temperature is perfect - not uncomfortably hot like an 80's hottub, more like a very warm pool. The lagoon itself is quite large. There's quite a bit of steam coming off the surface and very quickly you lose sight of all but a few people. At times I had the illusion of being completely alone while floating on my back looking at the stars.
Easily makes my best-of list of tourist experiences: bathing in the Blue Lagoon's thermal spring water mixed with sea water at night, in the winter, under starry skies with steam rising all around me.
The lagoon is a mix of cold and salty sea water and thermal spring water, heated by the same tectonic activity that made this island in the middle of the northern atlantic. Geologically Iceland sits on top parts of both the American and European continental plates. Bubbling hot springs can be found all over the southwest.
The water temperature varies a bit as you move around between cooler and warmer parts - very pleasant. I tried to stay in for as long as possible but I think I managed perhaps one hour.
Afterwards, I had dinner in the surprisingly cool Lava Restaurant. The big room appears carved into the lava stone with a view onto sultry streams, which made it look a bit like a Bond (as in 007) villain hideout.
I had a martini, shaken not stirred, because it just seemed appropriate
Icelandair offers the option of a few hours layover so you can pay a visit to the Blue Lagoon if you're making the transatlantic journey. Highly recommended.
the original "geyser"