First off, love the series, can't wait for the finale. I had to ask, have you read House of Leaves and if so, did it influence your idea of the tunnel in any way when writing? There are quite a few interesting parallels (obviously in completely original contexts) and I was just wondering if you were inspired at all by the book.
I really loved the stairs into nowhere in that footnote from the 1600s in House of Leaves, and I'm sure my mind decided to, in a totally different context, follow that path down. But the tunnel is also from a real-world place in Sintra, Portugal, and the spiraling of it is like the spiraling through my mind of oil during the Gulf Oil spill, when it seemed like the oil would just keep gushing out forever, and it was a terrible pressure in all of our minds....so, the real answer is that influence is a strange thing that manifests in different ways and recombines its DNA to form some creature that is itself totally unique.