Independent Venturer report on the threshold "Sewer Travel", written by Venturer Reginald on 11/27/2017 and compiled by Archivist Sarn for upload to the Archives.
Threshold Report

Photograph of Sewer Travel (Stage 1), located in Brussels, Belgium
Status: Active | Prevalence: Unique |
Overview
While on a trip to Brussels, I explored some of the more obscure liminal phenomena in the area (and by our standards, obscure means practically unheard of). Looking through the database, I found this entry, which at the time was little more than a handful of vague details, with some decades-old records that drew an unbelievable connection with the Cloaca Maxima of Rome. Everyone who's ventured here seems to disagree on where it goes, and there's no records of anyone reaching the actual end of the sewer before exiting the threshold. I endeavored to find out for myself where it really goes, and I wanted to go deeper than anyone has gone before.
This threshold is commonly known as "Sewer Travel," but I think it'd be more apt to call it "The Worst Transportation System Ever Devised," because it is — simply put — fucking awful. For anyone unlucky enough to find themselves traveling through, this threshold becomes the vilest, most uncomfortable experience possible. Torment is its purpose.
Description
At first, Sewer Travel seems almost normal, beckoning explorers to enter with the prospect of instant transportation. The threshold has five distinct stages, and each stage after the first has its own set of exits. If you can find the right exit, you can go pretty much anywhere. The sewer branches often, with different junctions that lead to hallways filled with exits, or you can progress to the next stage should you continue past every exit. The junctions are labelled, allowing for easy navigation, assuming you know where you want to go.
Each stage is more disgusting than the last, but also a significantly broader reach of exits. Once you're at the desired stage, you'll need to follow the signs until you find the correct exit. Not all of them look the same, but most are ladders up to manhole covers. It may take a while, but as long as you follow the signs, it is always possible to reach your desired destination, no matter how obscure. I tested this multiple times with precise locations, and I was always able to find the right exit within no more than a day of travel.
This stage has the appearance of a normal, rundown sewer. There are no obvious liminal properties.
Accessible from this stage: Nothing yet. You must travel deeper to reach a destination.