Bill, that is caused by overcharging. Are we talking about a battery in a vehicle? The battery may not be capable of properly accepting a charge, which may contribute to the swelling/bulging. It is probably gassing a lot and giving off a smell, too. Be carefull about acid overflow when handling this battery. You may need to thoroughly clean the box/holddown areas. At 4 years, this battery is getting pretty long in the tooth.
