Most of the container is steel but the floor is tongue-in-groove hardwood and sits on a steel cross-member framework, not a solid steel plate on the bottom. We can assume that the floor would be in direct contact with the soil unless some barrier is put in place, i.e. concrete, gravel, plastic, etc. While this is perhaps a manageable concern in an above ground, or partial-bury configuration due to the fact that there is at least some ventilation, full-bury implies no ventilation. This could yield warped or rotting floor, flooding, or even rodent/insect intrusion. So some additional barrier is needed.
Offered by Steve.
You may want to build extra structure on the inside, for example a concrete slab could be put on the floor of the inside.
Offered by Mike.