a) build themselves
Don't think so ace. Not only did hezzi blow through tons of iranian money building them, the majority of said tunnels are located in LEBANON OR THE GAZA STRIP.
There's the small problem of having a covert crew digging tunnels in hostile territory without anyone inquiring, including the person's home that serves as the secret place to enter and exit.
"Hey Rakim. Any idea what those guys with the earth moving equipment have been doing all this time?"
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"Oh, don't mind us. We're a secret hezzbollah team building an emergency water line. Please continue with your dinner. We'll put everything back when we're done. Please don't tell anyone we were here and don't open the secret door behind your bookshelf. Have a nice day. Salem hal lekem or whatever."