I believe it started with some articles that were done in '76, maybe '77 as well in publications like Rolling Stone I think it was, also it might have been mentioned in screenplays of Star Wars A New Hope that have been released in bookstores subsquently. Also the Return of the Jedi novelization has Obi-Wan make mention of something of it, of some lava or volcano, in the same part of the book where Obi-Wan tells Luke Uncle Owen is his brother.