MacBook Pro battery, with cells inflating like a balloon

Our laptop worked fine for its first three years, but this summer it started having trouble charging and it wouldn’t hold its charge. We bought a new battery, and it still refuses to charge most of the time. So once the new battery charged this weekend, we decided to charge the old one as a backup. This is what we found it had done in the last several months. (We’re recycling it right away.) In the picture, the normal looking cells on the left are perfectly flat, rather than puffed up like a balloon and buckling the enclosure.