It's normal for the iMac G5 to have surface lines on its back cover. These occur during manufacture of the enclosure and are called "mold lines." They appear on the back cover to the left of each port and the power button. Mold lines are not cracks and do not represent a weakness or defect in the plastic.
Note: Mold lines normally look very faint, so this picture has been darkened to make the line more visible.
