They're doing it wrong:
-
Back up a large propane tanker truck to the opening.
-
Drop a hose from the truck to the tunnel opening.
-
Use a bulldozer to backfill the tunnel opening with an adequate quantity of dirt/gravel.
-
Empty the propane truck into the tunnel.
-
Disconnect the truck and remove it to a safe distance.
-
Wait a little while to allow the propane to disperse through out the tunnel system.
-
With the end of the hose not pointed at your face, drop lit match into the end.
8.Turn towards the general direction that the tunnel appears to be leading and observe the geyser of fire and badguy erupting from the ground in the distance.