Hightmap in BFV is a RAW picture white to black in 256 steps untill patch 1.2 was released.
Now tunnels where introduced.
To be able to get into a tunnel you had to have a negative value of black!
So now RAW picture are in 512 steps, from white to black to
.
As soon as it is a negative value, that part will be transparent, both for eyes and feets.
To create this fenomena you can use tunnel tool.
This tool lower ground under level 0 and makes it transparent.
You can also use dig tool, but it will take forever.
This hole should be as small as possible, but not too small, just perfect.
Tired of a sniper using the same bush map after map.
Dig a hole for him under it. All he's going to be is a wet spot.