The hatchet-faced woman leads us behind the shoe store in this squalid suburb of Bucharest, Romania, and up cement stairs to a landing, where she tells us to wait. Clear plastic tubes run sewage along the sides of the stairwell and expel it onto the concrete courtyard below. The walls around me are toilet-green and the place smells of faeces and dead mice.

I have infiltrated an underground brothel in one of the city's most violent zones - Soseaua Orhideelor, in the quartier Basarab.

"Florin!" I hear the woman shout for her partner. Around the corner, a wild-eyed and aggressive man appears, his nose smashed, his arms lithe and muscled.

"I want to buy a girl," I explain.

"How much are you offering?" Florin responds, deadpan.

"That depends on what the girl looks like," I say. "Can we come inside?"

We walk to the second floor, where two women shout into a darkened room. A third woman emerges, clutching a girl, whom she identifies only as a blonde. She has bleached rust-coloured hair. Her head is shrunken, her nose flattened against her face. Mascara runs from pools of tears around deep-set eyes, cast downward at her bare feet with widely spread toes. Her hastily applied make-up cannot conceal the evidence of Down Syndrome. Lipstick is smeared beyond the boundaries of her parted mouth, and her flesh rolls out of the tight yellow tank top and shorts. Her captor holds her left arm so tightly as to hunch her shoulder. Below her right bicep are no fewer than 10 deep, angry red slashes.

I have been in a dozen seedy brothels on three continents, but I have never seen anyone in such a condition. I remember that I am wearing a wire, that I have to keep in character. I try to smile.

"For two months, two thousand euros," says Florin.

"Two thousand euros seems like a lot," I counter. "How about something else? A motorcycle-I can see that being about the right value."

"A car, maybe. Not a motorcycle. A good car."

"Only if I'm buying the girl for three months," I say. "And the car will come with 50,000 kilometres." The car I describe, I figure, might cost 1,500 euros.

"Ok," Florin says, revealing a stained grin. I shake his hand and return to the car.

"What the fuck was that?" asks my translator Alexandru. Still dazed, I don't respond.

"A blonde, huh?" he says. "Oh, yeah. Britney Spears."

It is off-colour, offensive. But I laugh. The alternative is to cry.

The most galvanising moments when I was researching my book into slavery were not when I went into dangerous situations, but when I re-emerged, like this, into safety, sometimes banged-up but generally unscathed. It was walking away from that suicidal young woman in the brothel, or the Nepalese families who'd begged me to take their children with me. Perhaps it was survivor guilt, but those were the moments - reinforced when I realised that authorities were not doing enough - when it hit me that the only ones who can make a difference are the people reading these words right now.

Today, there are more slaves than at any time in human history. The International Labour Organization puts the figure at more than 12 million, and credible estimates reach as high as 27 million. As John Miller, former director of the US State Department's Office to Monitor and Combat Trafficking in Persons, told me, "These victims don't stand in line and wait for a census to count them."

Just to be clear, a slave is a human being who is forced to work through fraud or threat of violence for no pay beyond subsistence. It is more than 200 years since the US and the UK banned the slave trade, and it's now illegal in most countries, but that hasn't shopped slavery - and human trafficking - exploding. Ruthless criminals have filled a void left by weak or corrupt governments, using fraud backed up with violence to ensnare the poor and isolated. Poverty has also played its part: 1.1 billion people now live on less than $US1 a day, meaning many families are desperately vulnerable to traffickers. Human beings have now surpassed guns as the second most lucrative commodity for crime syndicates of all sizes, netting around $US10 billion annually.

In the UK, slave traders still hold auctions in public places, including a coffee shop in the arrivals hall of Gatwick Airport, where a Serb pimp won a bidding war for two tearful Lithuanian teenagers in October, 2005. The girls had believed they were visiting a friend's father, who would host them for their first foreign holiday. Instead, the pimp brought them for £3000 a piece. As baristas frothed cappuccinos and other passengers killed time before their flights, the winning bidder led the girls, aged 18 and 19, away to the northern city of Sheffield. There, he immediately sold one again at mark-up and violently raped the other before forcing her into prostitution.

My interest in slavery stems from my childhood. I was raised Quaker, and at First Day school we learned as much about the escaped slaves-cum-abolitionists Harriet Tubman and Frederick Douglass as we did about Moses and Jesus. But it was while I was researching a story about chattel slavery in Mauritania, in northwest Africa, that I read the shocking statistic that there were 27 million slaves in the world. I decided I wanted to find them and show what their slavery meant.

So, over the next five years, I visited 12 countries and recorded interviews with more than 100 slaves, slave dealers and survivors of this heinous trade. During one month in Romania, I met a man who had made over a million dollars by selling human beings. I never paid for a human life, but I bore witness to the sale of human beings on four continents.

Read more of Ben Skinner's shocking exposé of modern-day slavery in this month's marie claire.