How prevalent are sewers that you can walk around in, actually?

I don’t know about older cities. But I’ve seen Chicago area sewer projects, and the sewer pipes are about 12 inches in diameter. There was never an extensive old-timey tall brick tunnel system

However, recent works have some spectacularly huge tunnels. For 40+ years there has been an ongoing project to build reservoirs to hold storm surges of runoff, and feed it slowly into the cleanup system.

The tunnels look like this and the reservoirs are old quarries like this.

They are usually “prevalent” in major cities. There will also be water mains large enough to walk through. But these large structures will comprise only a small percentage of the overall mileage of “pipes” and tunnels comprising the sewer system. A convenient way to visualize the sewer system would be the setup of the blood vessels in your body, with numerous smaller vessels “draining” into fewer and progressively larger veins.

A little off topic:

I don’t knowing common it is, but manholes are mostly found in big cities and can be for sewer and also other utilities.

NYC, and other cities up north have extensive underground tunnels, not always sewer though. NYC has many areas that pump out water from the underground, so it’s not terribly hard to find a wet tunnel to film a scene like that there if they are using a real life set.

I can remember walking in the storm sewers in Los Angeles in the 1950’s, standing up.

Nothing like that around here. Most of the pipes are 16” to 4’.

They’ve been prevalent in cities and even smaller towns for a very long time, because sewers generally need to be accessible for repair and maintenance workers. Pipes than can’t be reached by a human mean pipes that you need to dig up from above if something happens to them.

Most towns and other facilities tend to have underground tunnels, even if they generally are not exposed to the public.

The L A sewer system is legendary for those huge tunnels. Their storm drains as well.
Many don’t realize the difference between between storm and sewer drains. One leads to an outlet to a river or bay, the other to a treatment facility.

Do you mean storm and water drain sewers? (When I think of “sewer” I think of human waste.) When I was growing up we had a large storm drain that opened up to the creek behind our house. We could walk in there. Hunched over, but yeah. We explored.

Do you mean storm and water drain sewers?

They aren’t necessarily separate. Here the storm water also goes to the treatment plants. Googling says 772 American cities have a combined system.

Well, I just wanted to clarify that the storm sewer I traipsed around in as a kid did not contain human excrement.

I should be more specific – In a modern combined system, the upper part of the system is all runoff. That’s where the big tunnels would be. So no traipsing in sewage.

It meets the sewer later and they head to the treatment plant together.

Ah. IC. Thanks. When I was doing my traipsing it was in the late 60s. Was the system the same then?

I think it was the same in the 1960s. And not everywhere has combined systems. And I would assume there’s lots of diversity based on the age and density of the community, and the geography & climate.