There is another city hiding underneath Manhattan, and part of it is filled with 180°C steam. Here is what those white clouds and striped chimneys really are.
If you have ever walked through Manhattan and seen huge clouds of white steam rising from the road, you might have wondered what on earth is happening underneath New York City. Is the subway overheating? Is there a fire underground? Is New York secretly sitting on a volcano?
Thankfully, the answer is much less dramatic, but actually pretty amazing. Underneath the streets of Manhattan is a huge network of pipes carrying steam around the city, and some of them have been doing their job for more than 140 years.
## A city underneath the city
When we walk around New York, we normally notice what is above ground: skyscrapers, taxis, shops, subway entrances and thousands of people. But beneath the pavement is another enormous world of subway tunnels, sewers, electricity cables, water pipes, internet cables, gas pipes and, unusually, steam pipes.
New York's steam system began on 3 March 1882, when the New York Steam Company started supplying steam to buildings in Lower Manhattan. Today Con Edison runs it, with around 169 kilometres of steam pipes, about 3,000 steam manholes and around 1,800 customers. It is basically a gigantic underground central heating system.
## So what is all that steam used for?
Instead of every large building making its own heat, Con Edison makes steam at big plants and sends it through insulated pipes. Buildings use it for:
- heating - hot water - air conditioning and cooling - sterilising medical equipment - cleaning and laundry - controlling humidity in buildings and museums
The Empire State Building, Grand Central Terminal and the United Nations are among the buildings that have relied on it. On a cold winter day the system can send out around 10 million pounds of steam every hour. That is a lot of steam.
## But why does it come out of the street?
Steam in the street does not always mean a giant pipe has burst. The pipes are extremely hot, so if rainwater or water from another pipe drips onto them, that water turns into vapour. There can also be small leaks. On cold days the warm, wet air meets the cold air above the road and suddenly you have a giant cloud pouring out of Manhattan.
## What are those orange and white chimneys?
One of the strangest sights in New York is a tall orange-and-white striped pipe in the middle of the road with steam blasting out of the top. These are called steam stacks. They do not make the steam. They are temporary chimneys that send hot vapour up instead of sideways, across the people and cars. Con Edison uses them once crews have checked there is no immediate danger, while repairs are organised.
## How hot is the steam?
Very hot. The steam in the pipes is about 181°C, at high pressure. That is why you should never walk through steam coming straight out of a manhole or a crack in the road. Keep away and tell an adult so it can be reported.
## Why not put a boiler in every building?
Imagine every huge building in Manhattan needing its own enormous boiler, with space, fuel and repairs. Making steam in one place and piping it around is a bit like everyone plugging into the electricity network instead of owning their own generator. In Manhattan, where almost every square metre of space is valuable, that is a big advantage.
## New York has been doing this since the 1800s
When the steam started flowing in 1882 there were no phones in pockets, no aeroplanes and no televisions. The Brooklyn Bridge had not even opened yet (it opened the next year). Six months after the steam began, Thomas Edison's Pearl Street power station started supplying electricity nearby. The system has been rebuilt and modernised many times, but more than 140 years later, steam is still travelling under Manhattan.
So next time you see a white cloud rising between the yellow taxis, don't assume something has exploded. You might just be looking at one of the city's oldest pieces of machinery at work.
## Abby's Labs fast facts
- **Started:** 3 March 1882. - **Pipes:** around 169 kilometres under Manhattan. - **Manholes:** about 3,000. - **Used for:** heating, hot water, cooling, sterilising, cleaning and humidity control. - **Striped pipes:** temporary steam stacks that send hot vapour safely upwards. - **Dangerous?** Steam straight from a crack or manhole can be, so stay away and report it. - **Is New York sitting on a volcano?** No. But the real answer is almost as interesting.