So the secret is out. And unless you love crowds, skip the Blue Lagoon and come visit the Secret Lagoon. You will probably share the facility w/ a handful of people at most. Pretty minimal facilities and changing room/reception don't detract from the boiling water and stunning setting.
It is the oldest swimming pool in Iceland made in 1891 and its water holds at 38-40 Celsius (100-104 Fahrenheit) all year. It has wonderful natural surroundings and steam rising into the air, giving it an unbelievable magical feeling. It takes around 90 minutes to drive to the Secret Lagoon from Reykjavik.