How about corrosion resistant enclosure? And even it will be damaged (probability is probably low) you can have a sensor to detect that and replace the enclosure. Batteries burning underground should be less a concern compared to them burning on a wood wall of a house. In case of an incident circuit breaker will turn them off, and batteries will burn themself w/o damaging anything else.

If you're gonna have a fire a hole in the ground is probably just about the best place for it.