http://www.straightdope.com/mailbag/mhydrant.html
The report says that fire hydrants don’t freeze because the water main is located below the frost line underground.
Fact is, they CAN freeze.
Several years ago, a family very close friends with mine lost their house - burned to the ground - because the water mains feeding the local hydrants had frozen. The fire engines arrived promptly, but once they ran out, nobody could do anything about the fire but wait for it to burn out. Fortunately, they escaped uninjured, and insurance covered rebuilding.
It was early January of a particularly cold winter in northern Virginia. So while the hydrant mains are supposed to be “safely below the frost line” as the report says, they aren’t always at all.