http://revach.net/article.php?id=1467

Section: Moadim   Category: Chanukah
A Reader's Answer to the Bais Yosef's Question

One of our readers sent this beautiful answer to the Bais Yosef's question of why do we celebrate the Nes for 8 days.  If there was one Pach Shemen then we only needed to rely on a Nes for Seven days.

I once heard from Rav Avigdor Nebenzahl [Revach Editor: Rav of the Old city of Yerushalayim] based on the Ramban, who says that the point of miracles (nes galui) in general is so we should recognize that everything in the world is miraculous (even things considered natural). So the eighth night represents all nisim nistarim (hidden miracles of everyday life)

For more answers and reader discussion on this question you can go to the Help Wanted article "7 Days Miracle or 8"