Tunnels begins simply enough: after Dr. Burrows disappears mysteriously, his son Will, desperate to escape his tuned-out TV-obsessed mother and irritating sister, takes off with his best (and only) friend Chester to seek his father underground. It isn't long before they stumble, like Dr. Burrows did, on the Colony, a group of religious separatists who left the surface for the purer life underground, and their overseers, the demonic Styx. Soon, the boys find themselves trapped underground, attempting to find a way to foil the Styx's plan to unleash a powerful plague, Dominion, over the surface and reclaim it for their own.
What makes the Tunnels series stick out to me—especially in this post-Harry Potter middle-grade fiction world we live in—is that it isn't afraid to be grim and depressing; perfectly sympathetic and pleasant characters die frequently, and Will and Chester frequently find themselves confronted with the need to make difficult, no-win decisions. The morality isn't as grey as I would like, as the Styx are still presented as unadulterated evil with very little attempt in the three books I've read to give them a more sympathetic treatment, but the allies that Will and Chester find underground fill that anti-hero, good-isn't-always-good niche. The only thing that really grates me about the series is Will's tendency to call his sister Rebecca a "cow", as it comes off as a needlessly misogynistic insult and made worse by Rebecca's lack of depth as a villain.
It's the setting that's the most complex and fully realized part of Tunnels: dank, dimly lit caverns; bizarre flora and fungi; ruins of an ancient civilization and their Elysian realm at the center of the earth. It's tapping that subterranean nerve of mine—and given the popularity of this series along with games like Minecraft with middle schoolers, it's clear I'm not the only one with that particular nerve.