Amazon rainforest
Thousands of people moved to the rain forest to work collecting rubber. Most of these people were hired by rich rubber merchants. The rubber merchants loaned them money to come down the river and buy tools.  Each merchant’s collectors had to sell the rubber only to their rubber merchant at low prices and buy supplies only from them at high prices. That meant the collectors were always in debt to their merchant and could not leave to do something else. The rubber merchants quickly became very rich.