When I was a kid, a new shopping mall was built near my home and like any one my age, I found the construction site fascinating. I was particularly drawn to the massive drain system they put in place underground before they built the actual mall. The tunnels were big enough to traverse from one side of the site to the other and spread out to other drains in the area. I crawled around in those tunnels for a few weeks before the site was fenced off and my access ended.
Our urban and suburban landscape is littered with manhole covers and storm drain openings on every street. I read once in an online forum where a poster questioned whether or not these underground chambers should be considered for ad hoc shelters in an emergency like a nuclear event. First, we all need to identify the difference in underground infrastructure and then discuss why employing them in an emergency is not a good idea.
Storm and sanitary drains have been under humanity's feet for thousands of years. The underground drains and sewers of Rome and Paris are legendary for instance and most were built upon older structures dating back hundreds or even thousands of years ago. However, these drains and sewers served a very specific purpose: to transport excess water and waste AWAY from people and urban centers. They were not designed to be shelters of refuge although they have been used as such by different individuals and groups.
Storm drains - storm drains are designed by city engineers to carry large amounts of water underneath a city during rainy periods. While they are mostly dry and empty during dry times, they quickly fill with water to the point they are able to carry a fully grown person away through their twisting labyrinths. Storm drains comprise of street side openings as well as manhole access points on the streets. They then open underground into larger and larger drains which eventually empty into natural rivers or manmade ones like the urban canyons made famous in Los Angeles.
Sanitary sewers - these openings are seen on most streets and are the foulest of all. Sanitary sewers carry human waste from our toilets to waste centers where the waste is separated and the water cleaned for reuse in the water system. If you have ever seen one of these things backup, you know what comes out of them and its pretty gross. I would never go near one of these places.
Utility tunnels - these vary in size. Where I live, the phone company has several large openings to the underground where phone lines are run between buildings. While the manhole covers are large, what is below is cramped (room for one technician to work at a time) and not very safe as they run both electrical and phone lines through the same tunnels. There are often batteries and leaking chemicals in utility tunnels which make them more dangerous.
There are other utility tunnels in some older parts of the country which were used for transporting coal or fuel oil beneath the cities. And finally, there are utility tunnels which were used to connect subway tunnels for workers; these are completely different and might be useful.
The problem with storm drains and sewers is while they may protect people for a short amount of time, eventually, they will become inhabitable because of their intended purpose. Water and human waste will back up and flood these chambers driving residents out into the open. Even the famous storm drains beneath arid Las Vegas are emptied a few times a year because of rain and the homeless population below is sent packing to high ground.
If there is a nuclear event, like a dirty bomb, it is better to head for a real basement or parking garage for cover. The water and utility tunnels underground are not intended for shelter and could end up being a tomb rather than a haven.