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