It happened to my wife 20 years ago while starting her car. There was not one piece of the battery left that was over two inches long. When dealing with hydrogen gas, a single ignition source such as a spark or cigarette is all it takes.
I appreciate that yours was internal to the battery, but it's a good reminder that if you keep your batteries in an enclosed space, it's important to make sure that there is good ventilation so that an explosion won't occur within the confined space of the battery storage compartment.