2) How would you explain ‘tap a deeper vein’ in the context?
School is closed for Holy Week, but Mother is in an unholy frame of mind, due to the predicted collapse of the Mexican oil industry. According to Mr. P. T., production has fallen to less than a quarter what it was when the Americans first came in. They thought they were tapping a deeper vein, he said.
How would you explain ‘tap a deeper vein’ in the last sentence?