Why does steam percolate from below New York city streets?

Every time I see a scene of New York streets, there is always steam coming for the manholes.
Where does this steam come from?

Its an old centralised heating system due to its age there are lots of leaks. I was reading about it a little while ago it was started in the 1880s

Sweden has a modern version

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The cost of replacement would be huge, making it prohibitive I suppose.