Some people dig tunnels to make trade routes shorter or roads more direct; some dig escape tunnels; and some dig just for the engineering challenge. You can dig your own tunnel in the soil on your property. Remember the hazards if digging and never dig alone. Hand tools suffice for small tunnel projects; the only constant is shoring. Keeping the ceiling braced helps your tunnel to be more stable.
Don safety gear. Dig the tunnel out with a shovel, using plastic buckets to haul out the dirt. Take your time and make the dimensions of the tunnel large enough for your body to fit through without touching the sides of the excavation.
Loosen any compacted dirt and rocks with a digging bar. Do not dig the point of the digging bar into the top of the tunnel; this may cause the top of the tunnel to collapse.
Dig a vertical shaft as there must be enough earth above a tunnel to allow for it to be walked on from above without collapsing. The distance between the roof of the tunnel and the surface of the earth should be twice the height of the tunnel to be dug. For a comfortable environment to work in, give the tunnel a width of around two feet and a height of two to two and a half feet. This size is also stable and unlikely to collapse (as opposed to larger tunnels).
Dig a vertical shaft of at least six feet in depth, if following the dimensions of the tunnel in Step 1. Seven to eight feet deep is, of course, a safer distance. Make the shaft larger than the tunnel's width for getting supplies in and out of the tunnel -- around three square feet. At a point in the digging process it will be easier to use buckets to haul dirt out of the tunnels than to throw it. Tie a long piece of rope to a bucket and string the rope over a piece of wood or similar object at the top of the vertical shaft to haul the dirt out.
Cut the lumber with a circular saw. Make one piece of lumber long enough to fit across the top of the tunnel. Cut two lumber pieces high enough to use as braces on either side of the top lumber.
Place a board across the top of the tunnel. Place a board up each side of the tunnel to hold the overhead board in place.
Use a lump hammer, chisel or trowel to dig when digging outside of the vertical shaft and inside of the tunnels; a shovel will no longer be useful at this point due to size constraints. Use a pick to dig if the ground you are digging is too solid or contains a lot of rock.
Install shoring as you dig to make the process of tunneling safer. The spacing between shoring largely depends on the ground you are digging through and where it is relative to other tunnels. Denser ground will require less shoring, though erring on the side of safety is wise.
Nail the uprights to the overhead board with 16d galvanized nails. Place a new wooden brace every six feet along the entire length of the tunnel.
Install an air pipe before your tunnel gets too long. Use a flexible piece of piping at least one inch in diameter, cut with a knife to prevent the creation of bits of plastic, running from the entrance of the tunnel to along the sides or floor of the tunnel. Place a fan to push air through the pipe at the entrance of the tunnel.