Simple steps on how to bury your avatar below ground level.
Step 1: Rez a Prim.
Rez a cube or a cylinder.
Step 2: Stretch Prim.
Stretch the prim to its maximum length of 10m.
Step 3: Sit on It.
Right click on the top surface of the prim and select 'Sit here' to sit on the top.
Step 4: Rotate the Prim.
Rotate the prim 180 degrees so that your avatar is facing downwards.
Step 5: Move the Prim Down.
Move the prim downwards into the ground to hide yourself.
You can then move it sideways to 'creep' around without others seeing your avatar.
Step 6: Change the Texture of the Prim.
If the butt end of the prim shows up in certain places, you can change the prim texture to something totally transparent to make it harder to spot.