It seems like every big city has its own tale of underground tunnels. And the stories of what they were used for are often very similar to each other. For many west coast port cities, the stories often involve drinking establishments with secret traps doors. Bar owners would get a patron good and intoxicated, drop that patron through a trap door and into a basement, which led to a secret tunnel to the port. By the time the poor soul came to, he found himself shanghaied on a boat in the middle of the ocean.
Other stories include tunnels that ran underneath the street, from one side to the other. A gentleman might park his horse on one side, walk into a "legitimate" business, and take the tunnel under the street undetected to the other side where a den of vice would await.
These stories certainly persist in Tacoma. But Phyllis Harrison, who has a doctorate in Folklore, did extensive research on the legend of Tacoma tunnels. She says that while no evidence of secret underground tunnels in Tacoma necessarily exists, the folklore around these tunnels has strong historical roots involving the construction of the railroad and a strong anti-Chinese labor sentiment in Tacoma towards the end of the 1800s. And as that sentiment changed over time, so did the story of Tacoma's tunnels.