I was vacuming and noticed this last night. Cant imagine how this happened. Its bad huh???
Goes all the way to the lower back of it, and curves up and then back down to the floor. Im thinking, new toilet??? Its 13 yrs old. I bought it new. Gawwwd, I should HOPE I bought it new! Duhhh. LOL
Advice please?

Ceramic doesn't age. The worst case is a leak in the seal which has caused some wood rot. The softened wood won't support the toilet properly and will cause it to crack. If the toilet is sitting on a concrete floor it probably wasn't bedded properly in caulk causing a stress riser in which case you buy a new one and make sure it is supported evenly.

It appears to me that it was not installed properly and was most likely, overtightened on the anchor bolts causing the toilet to fracture. That's an ugly anchor bolt job also....bolts should have been trimmed off and plastic caps installed....versus painting the anchors. Looks a lot better and easier to keep clean.

I would recommend replacing it with a new one from Home Depot. A decent new toilet will be between $140 - $300 but they always have something on sale.

They will also provide proper installation for about $59 that will save you some headaches. (Price may vary by region)

Make sure you select a new one that has a good flush rating. They usually rate them with stars *** or numbers (higher number, better flush rating) and the sales person can help you.

Good luck!

Ahh, thanks guys ! I referred to the guy who reinstalled that, as
- The Angry Plumber - everything he did was messed up, and so shoddy.
Well, that explains it, I cant detect any moisture, but I will be checking it now.Guess it would rot out under where I cant see it though.
Lowes etc. not an option, its 60 miles from here :(
sigh.
Thanks again.