Grjótagjá is a small lava cave in Iceland known for its stunning geothermal hot spring. It's located near Lake Mývatn and has a fascinating history. In the past, it was home to an outlaw named Jón Markússon and later used by locals as a hot spring. However, due to volcanic activity in the 1970s, the water became too hot to use. Despite this, Grjótagjá gained fame when it was featured in the TV series Game of Thrones. Although you can't actually enter the cave or bathe in the hot spring, it's still worth a visit for its unique beauty.