I'm glad you posted these pics. They show what happens to SO-type cord when it is buried.
People seem to think "if it can go outside, it can go in the ground." We see here how burial- under even a little detritus- can degrade the rubber, and in a relatively short time. I have removed cords that had ALL the insulation completely gone because of this.
One way to identify the problem is the presence of dead vegetation- current leakage will kill grass, plants, etc. in the immediate area.
It should also be noted that your typical cast zinc "Bell" box will also quickly self-destruct when in contact with the ground.