Good idea: building tunnels under the docks to unload a ship if your city receives a lot of rain. Bad idea, using those tunnels to kidnap people and sell them as slaves to become sailors or prostitutes.

Historians say there used to be an intricate network of tunnels in Portland, Oregon, along the Willamette River. Evil people often used these tunnels to kidnap people from saloons or hotels above ground and sell them as slave sailors or prostitutes. The victim would awake a prisoner on a ship already out to ocean.
The crime was called "Shanghaiing" because Chinese people in Shanghai were sometimes kidnapped and thrown onto ships to become miners in Jakarta or the United States.

Apparently the "regulars" built trap doors in the street-level businesses to make it easier to drop a victim down into the tunnels!

Some tunnels still exist today, but none that connect with the waterfront.