For more than 150 years Port Charles has been a main location for the smuggling of contraband goods into the country -- specifically New York and the Northeast. The catacomb tunnels were originally dug under the city's Asian Quarter by The Triad. (Brad's biological family has run it for generations.)
Over time, the tunnel system spread over more of the town to handle the flow of illegal goods much of it coming over Lake Ontario from Canada. The heyday for contraband flowing into the city was during Prohibition (Booze of all kinds was illegal in the U.S. but legal in Canada) when the series of tunnels on Spoon Island (including under Wyndemere) were dug. Today smuggling is still a big business in Port Charles controlled by Sonny Corinthos and The Triad in a live-and-let-live arrangement, However, instead of using hidden tunnels the contraband is stored until shipment to New York City and other points in a series of waterfront warehouses.
A 1993 story-line saw Lucky Spencer hiding out in catacombs while hiding from Frank Smith. As a result it was revealed that after the city was first settled, an extensive catacomb system was built beneath the city.
In 2013, Sam is found and rescued by John McBain and Lucy in the catacombs under Wyndemere.
Robert Scorpio and Anna Devane decided to deal with Faison once and for all by leaving him trapped inside one of the catacombs under Wyndemere. However unbeknownst to them, Faison managed to escape.
In 2014, Heather takes Carly to the catacombs in Wyndemere. When Cameron, Emma, and Spencer discover Heather she tells them there's a Chupacabra hiding in the stables to keep them from finding Carly. Before Heather can kill Carly, she's discovered by Franco, whom she shoots, but is later stopped by Anna Devane.
In early 2018, Faison returns to Port Charles and kidnaps Liesl Obrecht, taking her to the stables at Wyndemere. When Faison discovers Lulu attempting to rescue Obrecht he traps her and gases the room leaving both to die. However, Lulu remembered the catacombs under the stables, where she and Liesl hid until they were rescued by Dante and Sonny.
At the beginning of 2019, Terry Randolph tells Oscar, Josslyn, and his parents that the treatment for his brain tumor did not work and that he will die in a matter of seven to eight months, but it might also go as early as six weeks. Oscar decides that he wants to live the rest of his life having fun. He tells Joss that he has not seen a lot of Port Charles and that he wants to see the catacombs. They get as far as getting to a cellar that is above them and he has another seizure. He is saved by Drew and Jason after Sam finds them trapped and is taken to the hospital for further observation.