This was a design used by the Viet Cong in Nam. It's very effective.
1) Shotgun shell
2) squares of plywood, about 2'x2' and 1/4" thick
Dig a hole big enuff for one piece of plywood to fit into. Drive a nail
through the center of one sheet of wood, and put it in the hole, nail
pointing up. Now, fill in dirt around this sheet, until it is mostly covered,
and only the tip of the nail is still exposed. It should still be below
ground level. GENTLY place the shotshell on top of the nail, and fill in dirt
around it, so it is supported on top of the nail, and held in place.
GENTLY place the second sheet of wood on top of the shotshell, and put leaves
and such on top of it.
When someone walks over it, they press the shotshell onto the nail, and it
blows right through the wood, and takes off their leg.