I bet I know what Toolman is getting for Christmas 
Yea... I wish....
I just read through the entire business opportunity and had a good chuckle.
I could also just imagine what would happen lighting that thing off anywere near town. Swat teams, bomb squad personel and Sheriff airships would swarm the area in minutes. Step away from the explosive device and lay on your belly, cross your legs while multiple AR's MP5's and maybe a Barrett 416 staring at you.
I see one fault with this product that I recommend Mike not renting this device. There is no timer on the gas mixture other than by guess or by golly. The user instructions includes a list of suggested times, depending on soil composition and estimated tunnel length for proper mixture. The fuel mixture timer is the operator using a time peice... stop watch etc. then after xxx seconds the operator shuts of the fuel mixture.
I could imagine someone leaving the gas on to long thinking if some is good...more is better... KA-BOOM. There should be a timer setting function that would possibly make it stupid proof.
And we all know you can't fix.....