Hard to tell from the photo, but it couldn't be gas bubbles because is not a plated zinc cent. If I had to take a guess it'd be die scratches, but a better picture might be enough to say more certainly.
Just die scratches. Most of the time, the die scratches when they are out in the open field areas, they tend to flatten quickly. When the die scratches are located at the edges of devices. They last a while longer there. Often times, they will remain and still be seen even when new die scratches are added to the die. But die scratches that are contained with in devices, like in the center of a mintmark, devices or like this one in the bay, They will last longer don't flatten as quickly and disappear as the ones in the open fields. That is what you are seeing.