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Ask CityLab is where we answer thorny questions about urban life. Email your Q's to AskCityLab@CityLab.com.

Dear CityLab: I often see manholes with steam coming out of them. What is that stuff?

Almost as representative of New York City as skyscrapers—though probably not as iconic—are the plumes of steam rising from beneath the ground. Sometimes they come out of the manhole covers; other times, they blow from giant white and orange tubes, like something out of a Dr. Seuss book. Walking by them evokes a mix of curiosity and disgust. After all, how clean can something be if it’s coming from underground?

But relax and take a deep breath, because that steam is usually made from water. In fact, some of the steam pouring out is the very kind used to clean the dishes in a New York restaurant, sterilize hospital equipment, and heat up cheese curds in artisan shops.

Where does it all come from?

Con Edison operates the world’s largest network of steam pipes. The steam system started with just 350 customers back in the late 1800s. At its peak in the 1920s and 1930s, the pipes had 2,500 customers across more than 100,000 commercial and residential buildings,The New York Times reported. Today, the network runs 105 miles, delivering steam to nearly 2,000 buildings throughout the city, including the Metropolitan Museum of Art, the Empire State Building, and the United Nations headquarters. “Had it not been for the steam system, the postcard skyline that you see of Manhattan would be totally different,” Saumil Shukla, vice president of steam operations at Con Edison told the NYT last year. “You’d be looking at every one of these high-rises with some type of chimney coming out of it.”

The company has five power plants throughout New York, where massive boilers produce steam that gets carried out of the plants, beneath the ground, and to Con Edison’s customers via a web of pipes. (You can see a tour of the system’s underbelly in the NYT video below.) A gallon of water can be converted into eight pounds of steam, according to Gotham Gazette. In the winter, as much as eight million pounds of steam can be generated each hour to keep buildings warm and heat up water. During summer, the steam is used to power cooling systems, supplanting some of the demand for electricity.

It’s a cleaner and more efficient alternative to having each building operate its own boiler. Speaking to Gotham Gazette, Con Edison spokesperson Joe Petta compared it to mass transit: “Which is better for the environment, 50 people riding to the city on a bus or 50 people riding 50 different vehicles?”

Should we be worried, then, that so much steam is escaping through the manholes?

Actually, much of the steam rising from the ground isn’t what is generated by the company. Rather, it’s vapor from when water, or other sources of liquid, falls onto the pipes and evaporates. (Remember, there is 350-degree steam traveling through those pipes). That’s why the plumes are extra big during winter, when snow and rain fall into the manholes. It’s also why, when you walk past the steam coming from the manhole covers, it’s not scalding hot.

But sometimes, steam can leak from the pipes. And when maintenance workers have to make repairs, they bring out the orange and white “chimneys.” Those not only prevent passersby from getting burned by the steam, but also guide the steam up so it doesn’t get in the way of traffic.

Is this just a New York thing?

A network of steam pipes isn’t unique to NYC, but its sheer size is pretty extraordinary. It’s bigger than the next nine largest steam systems combined, Shukla explained to NYT. And Gotham Gazette reports that Con Edison claims to have more than double the annual steam production of Europe’s largest system in Paris.

But such a large system comes with huge infrastructure and operating costs that can only be justified in densely packed places, which is partly why not every city has one.

So while it’s not the prettiest of NYC attractions, the city’s steam is certainly part of the package. And when you spot the white and orange steam stacks, it’s probably best to admire them from afar.

Top image: Saaton / Shutterstock.com​