The dark underworld of Bangladesh's clothes industry

Bangladesh's clothes industry has created its own distinctive landscape on the northern edges of the capital Dhaka.

From the sprawl of one-room houses and shacks where workers live, scores of multi-storey factory blocks jut into the sky.

Clusters of steel-reinforcing rods poke from their rooftops - in the hope of adding yet another floor of sewing machines.

It's a sign of what critics say is a boom gone too far, in the desperation to feed the West's appetite for bargain clothes.

The Rana Plaza which collapsed this week was another of these high-rise stitching stations, with the UK's Primark chain one of its customers.

Primark has said that it was "shocked and saddened" by the disaster and that it would work with other retailers to review standards.

But everyone involved in the industry is in the frame now - because it's had plenty of warnings before.

It's less than six months since a devastating fire at a plant that was making clothes for Walmart killed over 100 workers - Bangladesh's last worst industrial accident.

There had been some efforts to tighten up.

Humayun Kabir, CEO of DIRD Ltd, which makes nearly 20 million garments a year for UK retailers including Tesco, Sainsbury's and Next, says his customers were already "much more vigilant", making frequent random inspections of his production facilities.

"No child labour" signs adorn the entrances to many factories, promising compliance with the official ban on under-18s in the garment trade.

It's not clear though whether inspectors have also been looking for cracks in factory buildings.

And there's a whole underworld to the garments industry largely untouched by any checks.

'Open secret'

Dozens of sweatshop operations have sprouted up across Dhaka over the past decade trying to profit from the cheap clothes boom.

Bangladesh has one of the largest garment industries in the world

The BBC saw several back-street operations with few fire or other safety precautions and with people who were clearly children working on the shop-floor.

One of these sweatshop factories had made clothes destined for the UK in recent months.

It's an open secret that many higher standard factories often sub-contract orders to these sweat-shops to keep costs down and meet customer's deadlines.

It could just be a few thousand buttons or zips sewn on. Then the clothes go back to the 'good' factory - and the buyers may never know.

When it emerged that the Tazreen plant had been making clothes for Walmart, the US giant said it had no knowledge of this.

Whether any of the factories inside the Rana Plaza complex were involved in such practices is not clear.

Western buyers have been accused of turning a blind eye in the past, because of their interest in holding prices down.

The Bangladeshi government says it wants to improve conditions but worries about the knock-on consequences for the millions who now depend on the industry for jobs.

"The biggest human right is the right for survival," said commerce minister Ghulam Mohammed Quader in an interview before the latest disaster.

That sounds hollow now, as hundreds of Bangladeshi families grieve.

After this catastrophe, there is a lot more reflection going on over the real cost of cheap clothes.

As one Bangladeshi union organiser told the BBC: "You buy one get one free - but it's not really free."