I mean, this is the answer. The article's picture is not a picture of the site the article discusses, that's why. One thing I'll add is that the article says "

I mean, this is the answer. The article's picture is not a picture of the site the article discusses, that's why. One thing I'll add is that the article says "Many of the stones are buried deep in the earth." Presumably that's why nobody saw them until they looked pretty hard.