Dirty, Stained Socks!!!?

First off, I would like to mention that my husbands feet rarely see daylight. I have only seen his feet in the nude when he's in the shower or at the beach. Otherwise, he is wearing socks. His socks go into bed, just out of the shower, around the house, around the yard, and of course, out to the mailbox which is down the street. The bottoms of his socks are not only filthy at the end of the day, but when I wash them the black/gray footprint ABSOLUTELY REFUSES to come out. I'm pretty sure if I soak them in bleach any more than I have in the past, they will disintegrate. Do you my friends have ANY suggestions as to how I can get this type of stain out? Thanks in advance!! (And for those of you who LOVE to state the obvious, I have tried washing them and using bleach.