Tunnels

I know nothing about engineering tunnels, but it strikes me that they need 1) an exit and 2) air. If I were worried the tunnels were full of traps and murderers, I would simply seal the exits and airways. Maybe that is easier said than done? The other option - isn't there an ocean nearby? I'd set up a big hose and just fill the tunnels with salt water. Again...I'm not an engineer.,,