By Nita Bhalla, Anuradha Nagaraj, Rina Chandran – Thomson Reuters Foundation.

In the depths of India's illegal mica mines, where children as young as five work alongside adults, lurks a dark, hidden secret - the cover-up of child deaths with seven killed in the past two months, a Thomson Reuters Foundation investigation has revealed.

Investigations over three months in the major mica producing states of Bihar, Jharkhand, Rajasthan and Andhra Pradesh found child labour rife, with small hands ideal to pick and sort the valued mineral that puts the sparkle in cosmetics and car paint.

But interviews with workers and local communities discovered  children were not only risking their health in abandoned "ghost" mines but they were dying in the unregulated, crumbling mines.

In the village of Chandwara in Bihar in eastern India, a father's grief laid bare the ugly reality of the illegal mining that accounts for an estimated 70 percent of India's mica.

Vasdev Rai Pratap's 16-year-old son Madan was killed along with two adult workers in the state of Jharkhand on June 23.

"I didn't know how dangerous the work in the mines is. Had I known, I would never have let him go," said Pratap, sitting on a charpoy - a traditional woven bed - outside his home.

"They said it took almost a day to dig out his body after the mine collapsed. They cremated him without telling me. I didn't even see my boy before they set him alight."

Pratap, like other victims' families and mine operators, has not reported the death, choosing to accept a payment for his loss rather than risk ending the illegal mining on protected forest land that brings income to some of India's poorest areas.

The farmer has yet to received a promised 100,000 rupee ($1,500) payment from the operator of the illegal mine. No one from the mine was available to comment on his death.

Indian law forbids children below the age of 18 working in mines and other hazardous industries but many families living in extreme poverty rely on children to boost household income.

The Thomson Reuters Foundation findings were backed up by Nobel laureate Kailash Satyarthi's child protection group Bachpan Bachao Andolan (BBA) which documented over 20 mica-related deaths in June - including that of Madan and two other children - double the monthly average.

BBA discovered four children were killed in July.

India is one of the world's largest producers of the grey,  crystalline mineral that has gained prominence in recent years as an environmentally-friendly material, used by major global brands in the car and building sector, electronics and make-up.

A spokesman for India's Ministry of Mines said safety in mica mines was a matter for state governments who are facing  pressure from mining companies and social campaigners to licence more mines and crackdown on a black market that involves intimidation, worker exploitation, and child deaths.

BBA workers, who have been trying to stop child labour in Jharkhand's mica mines for almost a decade, said the latest deaths were just the tip of the iceberg, estimating fewer than 10 percent of mica mine deaths are reported to the police.

Officials from India's Directorate of Mines Safety were not available to comment on the number of child fatalities.

(Inside a Mica mine)

The National Commission for the Protection of Child Rights (NCPCR), a government organisation, found children as young as eight mining during a trip in June to Jharkhand.

"We didn't get any reports of children being injured or dying ... But it may be happening," said the head of the fact finding mission of NCPCR, Priyank Kanoongo.

Dutch campaign group SOMO estimates up to 20,000 children are involved in mica mining in Jharkhand and Bihar.

Investigations by the Thomson Reuters Foundation found children working in and around numerous mica mines.

In Giridih's Tisri area in Jharkhand, Basanti said her 10-year-old son Sandeep had been working in the mines since he was seven and the family in total earns 300 rupees ($4) daily.

"I know it's dangerous but that's the only work there is," she said, a metal dish partially filled with mica at her side.

In Bhilwara district in Rajasthan, boys as young as five were seen climbing down narrow shafts to cut mica with a hammer and chisel, while their sisters sifted mica on the surface.

 Dhanraj Sharma, a commissioner in Rajasthan's Labour Ministry, said he was not aware of child workers in the mines.

"They may be playing there, they may be doing some small things for the parents. That doesn't mean they are working," he told the Thomson Reuters Foundation.

 A recent surge in demand for mica has revived a flagging industry in India that dates back to the late 19th century.

Once boasting over 700 mines with 20,000 workers, the industry was hit by 1980 legislation to limit deforestation and the discovery of substitutes for natural mica, with only 38 reporting mines in 2013/14, according to Bureau of Mines data.

Renewed interest in mica from China's economic boom and a global craze for "natural" cosmetics has sent illegal operators scurrying to access the hundreds of closed mines.

Up to 10 percent of mica is used in make-up, sparking global headlines, but campaigners want other major buyers to get involved to end child labour and fund schools and training.

In Koderma's Dhab, one of about 45 villages where BBA is working, Pooja, 13, proudly shows off the run-down school she has attended for almost two years since she left mica mining.

"I like going to school," Pooja. "The mining was dangerous .. we were always looking up thinking the earth and rocks might fall on us. It happened to me once, but I managed to get out as my friend Munni helped me."

But any change is too late for Pratap who last saw his son in April when he left the village in search of a better life.

"He told me he was going to do something with his life and I was happy for him so I let him go," said Pratap. "How could I know the work he was doing was going to kill him?"