That is one worn-out piece of rubber.
The other day I had to do something incredibly rare for a mountain biker. I had to change a tube that didn’t have a hole in it. It was just worn out. This is the mountain biking equivalent to actually using up an ink pen before you lose it.
The rear tube in my mountain bike has been the same one for the better part of two entire seasons. The front has been in longer, but I expected that. It’s a True Goo tube, and those things are darn near indestructible. But the back was just a plain-jane tube. I don’t even know what brand it is, and in the condition it’s in now, I’m not sure I could find the markings. It probably has somewhere around a thousand off-road miles on it, and was never punctured, never flatted, and only recently started seeping air.
I had every intention of trying to patch it, until I took it out of the tire and saw the shape it was in. It’s incredible the thing was holding air at all! Most used tubes around here get cut up for re-use in some project, but I think this one might just be retired as a decoration.