In a scene that could almost have come out of an Ancient Art Archive presentation, Jon and Daenerys view ancient art in a cave.
"They were right here, standing where we are standing..."
The art bit starts around 2:40.
Now to my eye, those engravings look a little too fresh to be ancient, and Jon Snow's motives might not make him the most objective observer. If I were Daenerys I wouldn't assume that they were genuinely old until the Uranium / Thorium or Carbon 14 dates came back and were peer reviewed.