Army 1(the defending army) has the city of Phoenix, Arizona.
Army 2(the attacking army) has an underground tunnel under the city. The tunnel circles the city under the outskirts, has a few spokes (between 4-10, haven't decided yet)going towards the center where they plan to spring from the ground and attack Army 1, and one long tunnel that goes back to the entry point. So, wagon wheel on a stick.
Army 1 has learned of this attack, and needs to stop it. Army 1 also has a high ranking Army 2 official willing to work with them(as long as the plan isn't blatant suicide). Army 2 thinks this officer was captured, and if he appears in the tunnel after 'escaping' he can try to do something.
- Army 2 officer/Army 1 spy cannot die(or at least thinks he won't).
- Assume modern tech, although it is slightly in the future but weapons are mostly the same.
- Phoenix needs to not collapse in the process(I think that rules out big explosives).
- The tunnel is too big to gas.
- Solution preferably has some kind of explosion for a fun story.
- Army 1 cannot enter the tunnel without death happening to them(unless the fortifications were somehow neutralized)
- Two days to prepare, then three hours to finish plan.
- If Army 1 just blockades the exit, Army 2 will just come up somewhere else. If Army 1 keeps blocking exits, Army 2 will eventually just pack the tunnel with explosives and blow up the city.
- Minimal loss of Army 1 life
- More upon request
My first idea was to gas the place but that's not interesting, and big explosives would just bring down the city, and although that would look cool I need the city later. I thought maybe Army 2 officer/Army 1 spy could run through and drop methane into the tunnels, then get out and Army 1 could light a match and use up all the oxygen/burn the soldiers, but idk if that would bring down the city. Any suggestions?