Symbolism, Imagery, Allegory
The tunnel that runs underneath the Houses at Jellicoe School is a testament to Webb's incredible attention to detail. It's not enough for him and his friends to simply "have a war" (17.28). No way—they have to have a genuine tunnel just like the Nazis. As a result, Webb and Fitz spend most of their time the year after the war's creation digging a long hole in the ground to fulfill this purpose.
Like the Prayer Tree and Chairman Meow, the tunnel is one of those things that heavily connects the past and the five to Taylor's life in the present. At first, she sees it as something mythologized by the likes of Jessa, who's prone to theatrics and keeps saying her father always said the tunnel was real. Once she makes the connection that Jessa is Fitz's daughter, though, the story gains loads of credibility. And just in time, too—because Jude confirms the tunnel's existence, they're able to rescue Jessa and Chloe when they disappear into it to escape the fire at the end of the book.
The creepy thing about the tunnel, though, is that Tate actually foretells the incident with the fire without even knowing what she's saying. "I'm all for the tunnel," she tells Webb. "It could save our life one day" (17.18). Maybe the tunnel doesn't literally save the five's lives, but it does save the life of Fitz's daughter, his only legacy, though they couldn't have known it at the time.