A note on sewers

I actually spent most of my adult life working in Wastewater.  While we are probably dealing with 'fantasy' sewers which often have lots of endless cinematic windings, walkways and whatnot, the reality would be that this thing does not actually have very much room to hide.

Even in a large sewer system, I'll use Atlanta, Georgia as an example as that's one I'm very familiar with, the vast majority of the system is made of pipes less than 2 feet wide.

Now, Atlanta's system is huge, and it does have some very large sections.  Especially as you get closer to either the wastewater plants or whatever body of water they discharge to in the case of storm water systems they can get large enough to stand in.  These are not super common though.  The water is collected from all over, and brought somewhere for treatment, and most of those lines need not be large at all.  Interestingly, Atlanta's system is a good bit over 100 years old, and the oldest sections started as uncovered ditches that were later lined with brick, and later after that covered with brick.  Some of those areas have the traditional fantasy look.

Also of note, the closer you are to areas like parks or natural features the lines are smaller, because buildings, roads and sidewalks are all impermeable and don't allow rainwater to soak into the ground.  Those areas have to have sufficient drainage or you will get flooding from even mild rainstorms.

Sewers are in general not pumped, they flow by gravity to minor collection points called lift stations.  The sewer runs at a downward grade until you get to what is essentially a well with a couple of pumps at the bottom (always at least 2 for redundancy, these can never actually stop working), and the water is pumped up into another sewer line that runs down to the next lift station, and so on until it finally reaches it's ultimate destination.

In neighborhoods these lines are rarely bigger than a foot, and often only 8 inches or so.

Modern sewers have separate lines for sanitary sewers that go to wastewater treatment plants, and stormwater sewers that take the runoff from streets and go to an overflow facility (these can be quite large) that catches the first few hundred thousand gallons for treatment at a wastewater facility due to motor oil and whatever else, while the rest of the runoff is simply screened for grit, leaves and general trash before being sent along to the local river or whatever as it's pretty much just clear water.

In a modern, non-fantasy sewer this thing is either hanging out at a break that has eroded into a small chamber adjacent to one of the bigger lines, or its actually at an overflow station---seriously, I've seen those things in person and the catch basin for the first flush would fit a smallish apartment building.  Ive seen a Volkwagon wash up on the barscreen of that place.  Stormwater drains off an area with lots of impermeable surface can be large indeed.

Atlanta also has some huge, and I do mean HUGE tanks burrowed out from the bedrock under stone mountain to hold water during bad rainstorms that gets pumped back into the sewer system for treatment so as to not cause flash flooding.  You could toss several largish apartment buildings in one of those, of which there are 4, and they would rattle with room to spare.

Those facilities do get monitored, especially in weather when they are being used...  but rarely did we actually walk out to inspect the furthest reaches of the catch tanks, and we would occasionally find small communities of homeless had entered the facility, sometimes with tragic results when it came online and filled.

"Fantasy" sewer systems are actually based on those used in parts of Europe, particularly the roman ones that were developed something like 3000 years ago. Perhaps the most famous one is in Paris, which is only around 700 years old, but is huge and is actually a popular tourist destination. It is also still used today (although it has had updates over the years to support the increase of population and protect the seine river).

The sewer here ranges from areas you could play football in to pipes a rat would have trouble going through. But as a general idea, a google image search of "french sewers" is more or less what you find here.

The Paris sewers are hardly a modern system.

It is used, but without doing any research on it I'd bet it's mostly a storm system at this point.  I'd not think an open sanitary sewer would attract many tourists.  I could be wrong though.

There are small sections of the Atlanta sewer system that look like that, but for the most part you could not use it to travel much of anywhere.

I assume the parts they tour are not active, or at least not smelly, but it is a popular tourist destination. I have never toured it myself, but I know my parents have.

I was thinking that even in large pipes there isn't a really good hiding place, so no matter the size I assume that we are looking for a place where it can make its nest...like a pumping station or something...

My Money would be on a break in the pipe.  It happens all the time.

The rotten egg smell comes from Hydrogen Sulfide Gas, which in turn becomes Sulfuric Acid.  This causes the pipes to degrade over time above the water line.  Once there is a hole, water and more acid seep in, and generally the earth around the pipe is much more vulnerable to erosion, so over time you can get big caves.  Roots, and sometimes settling can make holes or help widen exising ones too.

Systems like he's describing in Paris have the same issue, plus outright vandalism and such since people can easily get into them.  It's actually why manhole covers are made of plate steel---to make them heavy enough that people don't like messing with them.