A ladder propped against a stained wall leads up into a dark passage on the second floor of an Indian brothel, lined by a series of locked doors. Hidden inside are tiny cubicles, stashed with sex workers' clothes, blankets, cosmetics and condoms.
The barely-lit passageway meanders along, intersected by many other dank corridors, and arrives at a trap door, which swings open to reveal another secret space, rarely seen by clients or outsiders.
"They are actually meant to deceive and hide," one sex worker said quietly.
"A person can get lost and then simply disappear."
Trafficked young girls are being "broken into prostitution" - and hidden from the law - behind a maze of passages and secret cells in crumbling brothels across New Delhi and other major cities, campaigners say.
Of an estimated 20 million commercial prostitutes in India, 16 million women and girls are victims of sex trafficking, according to campaigners.
Thousands of children, largely from poor families, are lured or abducted by traffickers every year, and sold on to pimps and brothels who force them into sexual slavery.
"These tehkhanas (hidden cells) harbour minors and have also become an escape route for them when there are raids," said Swati Jai Hind, head of the Delhi Commission for Women, which has rescued 57 girls this year.
"We get specific tip-offs about children being brought here but when we come for rescue, we sometimes find no girls - they vanish."
The government has introduced a number of measures to combat sex trafficking - from strengthening laws to boosting social welfare schemes.
But reports of young girls being sold for sex and hidden in labyrinths are rising, campaigners say.
"There are increasing cases where girls are describing life inside these dark and dirty places," said Rishi Kant of the anti-slavery charity Shakti Vahini.
"We were part of a rescue where a seemingly regular cupboard led to a hidden passage from where girls were found. Urgent action is needed."
Hidden in bunkers
When policeman Prabir Kumar Ball started investigating a missing persons complaint in India's eastern West Bengal state this year, he thought it was a routine case.
But the search for a teenage girl led him to the brothels of New Delhi and Agra, a popular tourist destination some 200 km (124 miles) south of the capital and home to Taj Mahal.
"The brothels in Agra had bunkers, just like the ones found along international borders," he said.
"We had to break into them to rescue the girl. We found six others hidden in these bunkers. Rescuing them was like going to war," he told the Thomson Reuters Foundation.
Ball said the traffickers take girls from West Bengal to Delhi safe houses, then sell them on to brothels in other towns.
The arrest of a couple from Delhi in November dismantled one of the region's biggest trafficking networks and gave "a rare insight into how bunkers and tunnels are used to hide young girls when police raids happen", he said.
Many trafficked young girls end up on the congested streets of New Delhi's largest red light district, known as GB Road.
Dimly lit staircases, next to ground floor hardware stores, lead up to hundreds of multi-storied brothels. Pimps haggle with customers, older women solicit and younger ones watch quietly.
As exchanges are agreed, customers enter the brothels. They are led to small, windowless rooms and the doors are closed.
"Nothing in this place has changed since I was brought here 20 years ago," a sex worker said as she applied make-up and got ready for clients.
"It was a dirty place when I came and still is. The maze of rooms, the way deals are struck and the plight of the women stuck here is frozen in time."
More and more survivor testimonies are providing evidence about brothel layouts and the extent of exploitation in them, spurring many agencies to push for their closure.
West Bengal's child welfare committee ordered the police in May to demolish "hidden places" in GB Road brothels, after listening to the testimony of a rescued girl.
The Delhi Commission for Women has also written to the police and civic authorities, demanding they identify and seal the cells and passages.
"No action has been taken," said Hind.
"We are working on a database of people who own these brothels and are determined to see they are shut down."
