In Arma, buildings and other objects are frequently modelled with walls and stairs that extend below flat ground level (z=0), to allow the object to be placed on steep terrain and still work.
OBS:
If the player goes prone and deploys a weapon near a building with such subterranean stairs or other platforms, the player can snap to these underground surfaces, placing the player partially or completely underground.
EXP:
The player should not be able to deploy in the ground. {F27762}