Grjótagjá is a cool lava cave in Iceland near Lake Mývatn, known for its hot spring inside. It used to be a secret bathing spot until the water got too hot from volcanic eruptions in the 70s. You might recognize it from Game of Thrones, where Jon Snow and Ygritte had a steamy scene. Even though you can't take a dip anymore, it's still worth a visit for the unique vibes and stunning scenery.