Tortoise belly shell split

by Jane
(Michigan)

We have a baby suclata tortoise as well as a 3 year old one. We got the baby in December of last year, and he seemed in good health. About a week or so ago, we realized that his belly shell (I don't know what this is called) was split and bleeding tiny a bit. We soaked him in clean warm water (as usual) and it seemed to sort of seal shut in the next day or so. However, I fear that the worst has happened. When we went to soak him today, he seems unresponsive. It is not clear if he has passed or not, as yet.

Does anyone know why the lower shells split? We do not believe that it had anything to do with improper or rough handling.