Grjótagjá is a small lava cave situated in Iceland's north. It is one of the most popular tourist destinations in the area and has become famous for its natural hot spring located inside. The cave is located in the Mývatn area of northern Iceland, close to the Krafla volcano. It was formed during a volcanic eruption thousands of years ago and is around 30 meters long and 15 meters wide.

The water temperature inside the cave can vary from day to day, but it usually ranges from 37 to 40 degrees Celsius, making it a popular spot for swimming. However, due to geological instability, the water temperature can sometimes become too hot for swimming, and access to the cave may be temporarily closed for safety reasons. Visitors are advised to check with local authorities before attempting to enter the cave.

Grjótagjá gained worldwide fame after it was featured in the popular TV series Game of Thrones. The cave was used as a filming location for a scene involving the characters Jon Snow and Ygritte in the third season of the show. Since then, it has become a popular tourist attraction for fans of the series, as well as nature enthusiasts visiting Iceland.