My uncle practically ran an underground factory, disregarding the very serious legal consequences.

It had been designed by one of the underground factories, apart from all human contact.

All those underground factories, where no prying eyes can see.

Was it some kind of air-shaft, for a tunnel, or an underground factory?

Down in your underground factories behind our lines.

They are dumped into an underground factory where some zombies attack them.

The amazing thing about it was that what went into the food packages is chemically prepared in underground factories.

"We were put to work there building an underground factory," he said in an interview.

Sent each morning into caves, they dug out bunkers for an underground factory.

This might be possible if they were coming out of underground factories.