By Siddharth Kara, Special to CNN

Editor’s Note: Trafficking expert Siddharth Kara is a Harvard fellow and author of the award-winning book, "Sex Trafficking: Inside the Business of Modern Slavery." For more than 15 years, he has traveled around the world to research modern-day slavery, interviewing thousands of former and current slaves. Kara also advises the United Nations and governments on anti-slavery research and policy.

A bonded laborer named Haresh in West Bengal, India, once described to me how he took a loan of approximately $110 from the local landowner to get married to his beloved wife, Sarika.

Two decades later, Haresh told me, “My entire family is still in debt to the landowner. Sarika and I work in the fields, my sons and their wives work at the brick kilns. One day my grandchildren will work for the landowner. There is no way to repay these debts. We will only be free when we die.”

Haresh had no real sense of what his outstanding debts were. Since his initial loan, he had taken numerous loans from the same landowner for basic subsistence, medicines, repairs to his hut and other reasons. He was also charged interest that often exceeded 100% per year. Destitute and isolated, Haresh could not access any other source of credit. He and his family were forced to work 14 or more hours a day by the landowner, almost every day of the year, with barely enough food and water to survive.

Haresh’s story is an example of the millions of bonded laborers across South Asia. Like Haresh, many bonded laborers have been in bondage for much of their lives. Others enter in and out of bondage several times; and still others enter in and out of bondage every year for seasonal industries, such as agriculture and brick-making. These individuals take loans and try to work them off, but due to deeply exploitive manipulations by their exploiters, they end up toiling against these small debts for years.

It is important to understand: bonded labor is a form of slavery prohibited by international and South Asian law. However, bonded labor remains an ever-evolving, highly complicated mode of labor exploitation that persists in broad daylight.

At its essence, bonded labor involves the exploitive interlinking of credit and labor agreements that devolve into slave-like exploitation due to severe power imbalances between the lender and the borrower. The system persists due to poverty, absence of alternative credit sources, a lack of justice and rule of law, and social acceptance of the exploitation of minority castes and ethnicities that has been prevalent in South Asia since Vedic times.

Debt bondage is not historically unique to South Asia. It was a common mode of feudal labor exploitation across much of the world centuries ago. However, a mix of social revolution and transition to industrialized market economies largely extinguished bonded labor throughout Europe, North America and East Asia. No such revolution ever took place in South Asia.

As a result, I estimate there are approximately 18.5 to 22.5 million debt bondage slaves in the world today, almost 90% of whom are in South Asia. This makes bonded labor the most expansive form of slavery in the world, with approximately six out of 10 slaves being bonded laborers. (Related: The challenges of counting a ‘hidden’ population)

Bonded labor is also an active contributor to the global economy. I have documented hundreds of bonded laborers in more than 20 industries, such as rice, tea, frozen fish and shrimp, carpets, cigarettes, fireworks, minerals and stones, gems and apparel. I have also traced the supply chains of these products from the point of exploitation in South Asia to retailers in the European Union and the United States.

Beyond the global economy, bonded labor may also present risks to global security. My most current research indicates there are increasing security risks associated with the suffering, poverty, and exploitation that feeds into debt bondage. Extremists in Pakistan and India have begun to recruit amongst current, former, and potential bonded laborers with promises of income, stability, and a way of fighting back against the governments that have consigned them to abuse and exploitation.

Bonded labor is a relic of history that should have long ago been eliminated from South Asia, but greed, corruption, and government ineffectiveness allow this caustic mode of exploitation to persist well into modern tines. In order to ensure basic human rights, guarantee untainted global supply chains, and protect international security, the forces that promote bonded labor must be tackled immediately.

More from Siddharth Kara on The CNN Freedom Project