There are, shockingly, more people in slavery today than at any time in human history - but campaigners think the world is close to a tipping point and that slavery may be eradicated in the next 30 years.

The estimated number of people in slavery - 27 million - is more than double the total number believed to have been taken from Africa during the transatlantic slave trade.

Ship records make it possible to estimate the number of slaves transported from Africa to the Americas and the Caribbean, from the 16th Century until the trade was banned in 1807 - and the figure is about 12.5 million people.

The figure of 27 million slaves today comes from researcher Kevin Bales, of Free the Slaves - who blames the huge figure on rapid population growth, poverty and government corruption.

We know there are many more slaves than homicides in the US Kevin Bales, Free the Slaves

Many people still think of slavery as a thing of the past, but it exists in many forms, on every continent - ranging from sex and labour trafficking, to debt bondage where people are forced to work off small loans.

"I often think about a quarry slave from North India," says investigative journalist Ben Skinner, who has travelled all over the world documenting cases of slavery.

"I could go in at night and interview him, so I asked him why he didn't run away. It was because he feared the extraordinary violence of the quarry contractor who held him to a minuscule debt.

"In his world, the contractor was god. He was not only the taker of life but also the giver of sustenance. When we look at why slavery has persisted we have to look at breaking those cycles of dependence."

Slavery in history Not all slaves were from Africa. Slavery existed among some Native American groups and in some Asian countries, and Europeans were sometimes enslaved by the Ottoman Empire. Russian serfdom can be seen as a variant of slavery. At the time of emancipation in 1861, there were more than 22 million serfs. Slaves were common in the ancient world - the Greeks, Romans and, Egyptians all held slaves. Professor Peter Kolchin, historian Was slavery already endemic in Africa?

Skinner says that many of the slaves he met in India had never known a free life. They came from extremely isolated communities, and were not aware of their basic universal rights.

But while developing countries have the highest number of slave labourers, developed countries with strong human rights laws "fail to resource the law enforcement to deal with the problem in comparison to virtually any other law", says Bales.

Barack Obama recently painted a portrait of contemporary slavery.

"It's the migrant worker unable to pay off the debt to his trafficker," he said. "The man, lured here with the promise of a job, his documents then taken, and forced to work endless hours in a kitchen. The teenage girl, beaten, forced to walk the streets."

The US government spends billions on tackling homicide, Bales argues, but only a fraction is spent on slavery "even though we know there are many more slaves than homicides in the US".

In Europe too, victims of slavery cannot always rely on the law to protect them. Anti-trafficking charity Stop the Traffik cites a case where a girl was returned to Hungary after being trafficked abroad. Upon her return to supposed safety, she was raped and returned to her traffickers.

Image caption As well as being transported out of Africa during the transatlantic slave trade, slaves were also captured and sold within Africa

Image caption Brazil has an official "dirty list" of employers using slave labour. 2,700 charcoal camp workers have been freed from slave-like conditions in recent years, according to Greenpeace

Image caption A child works at a brick factory near Lahore. There are thought to be 3.3 million child labourers in Pakistan