Q: why the steam rises from manholes in NYC?

I've been asked about the same question hundreds of times, but I never knew the correct answer. I always told them it's the steam of hot water pipes. Few days ago, one of my friends came to visit me and asked same question again. I tried to find the answers from yahoo answers. Somebody replied that it's Ninja Turtles eating pizzas underground @@. In the end, I tried google answers and I got an answer which makes more sense to me. The following is the link of answer form google answers. If interested, plz click this link.
http://answers.google.com/answers/threadview?id=90626