No coal mines in this part of the state. There are a fair number of natural
limestone caves, where mushrooms were once grown, but they are now grown in
climate controlled mushroom houses, which are partially underground. The
industry is centered here mainly because it started here and mushroomed from
there.....It's a tricky way to make a living; the canned mushroom market,
which used to be a staple, has withered due to foreign competition, and
producers have had to switch to marketing fresh, specialty mushrooms.

Here's an article, long and slightly technical, but quite interesting. It
goes into detail about compost production. Many of the mushroom farms are
now surrounded by encroaching suburbs, with residents that complain about
the smells.
http://aginfo.psu.edu/PSA/s99/mushroom1.html

Cheers,
Sue

Zone 6, Southcentral PA

"Pat Meadows" wrote in message expensive in the
stores!
Yep. It's my understanding that almost all the mushrooms
grown commercially in the USA are grown in Southeastern PA -
and I believe they are grown in old coal mines.

The Delmarva Peninsula is a very large chicken/egg producing
area and it wouldn't be far to truck the chicken manure
component to the mushroom-growing area - so maybe that's
where they get the chicken manure part of the compost.

Pat