There's one in the back of the hills if you're looking towards lava it's to the left of the castle behind those mountains I believe, there's an entrance right in front of the castle in the swamp, so far that's all I know
There is one right under the plateau Hythe castle. If you fly down clif away from cartels you fill see tons of metal. Around 30 nodes. Then you will see a path leading into a big hole.