(see: https://www.historic-uk.com/HistoryUK/HistoryofEngland/Barbary-Pirates-English-Slaves/, to start. cities that were frequently raided by muslim pirates seeking white slaves included london, dublin, lisbon, marseilles and rome, to name a few.)
our entire concept of using religion to form a basis for slavery was adopted from the reconquista, when spanish warlords adopted arab slave practices in reverse and started enslaving non-christians in response to the arab enslavement of non-muslims. it wasn't until he 18th century that american slavery became about race instead of religion, as a response to the christian abolitionist movement, that wanted to free the slaves by converting them. the slave owners had to find some other excuse to not release their black slaves.
so, they can't have these tunnels; they're just havens for organized crime. if this round of kidnapping is unusual, it's an algorithm that you should expect to repeat itself.
there's talk of flooding them with seawater, but i don't think that's the right approach, either. if i were israel, i would want to take control of the tunnels, clear them out, keep the ones that are useful and fill the rest in with cement.
not seawater; cement.
cement is a strong foundation to build over.