Grjótagjá Hot Spring.

image
Photo by Andrés Nieto Porras on Flickr | Copyright: Creative Commons.
The beautiful natural hot spring is located near Lake Mývatn, Iceland and has been used as a bathing spot for locals for as long as anyone can remember.
The waters in the shimmering cavern are heated by volcanic activity deep in the earth, making it the perfect spot to take a dip in the freezing Icelandic winters.
In addition to being a popular bathing spot, the cave served as a redoubt for 18th century Icelandic outlaw Jón Markússon.
In 1975, when one of the nearby volcanoes began to erupt, the water temperature rose to dangerous levels, and while the cave in the fissure could still be visited, people were no longer permitted to bathe in the hot waters.
Lately the temperature of the waters has begun to fall once again and people are being allowed jump in the waters once again.
The cave is beautiful and fantastical that it actually made its way into the fantasy canon when it was used as the secret sex cave where Jon Snow sealed his deal with Ygritte, in the Game of Thrones season three episode, Kissed by Fire.
Great job Iceland, you no longer simply seem like something out of a fantasy novel, you actually are.
Source: Grjótagjá | Atlas Obscura