The exploitation of tens of thousands of migrants used to grow salad vegetables for British supermarkets has been uncovered by a Guardian investigation…in southern Spain...Charities working with illegal workers during this year's harvest claim the abuses meet the UN's official definition of modern-day slavery, with some workers having their pay withheld for complaining…The Guardian's findings include: Migrant workers from Africa living in shacks made of old boxes and plastic sheeting, without sanitation or access to drinking water...Wages that are routinely less than half the legal minimum wage...Workers without papers being told they will be reported to the police if they complain.