Zaheda is worrying about her daughters. At 20 and 18, respectively, Gulsar and Abeda should be married by now, but life is not as it should be. It hasn’t been since they fled their homes with a dozen other family members almost a year ago. “Marriage is what is expected of women,” 40-year-old Zaheda says. “It is tied up with honour; not being married brings shame.” A man she knew in Myanmar wants them to marry his sons, but she can’t pay the dowry he’s hoping for.

Zaheda and her daughters are clustered together on the floor of a shelter in the world’s largest refugee camp, in the Cox’s Bazar district of south-eastern Bangladesh. Formerly two separate camps, it now holds over 700,000 Rohingya people who have fled Myanmar, spread across the hills in row upon row of tarpaulin and bamboo shelters. Four generations of the Begum family live here: 70-year-old Nur; her 50-year-old son Anwar and his wife Zaheda; their 10 children, aged from two to 25; and Nur’s great-grandson, Sultan, aged two.

The Begums left their home in Rakhine state, a low-lying coastal area in western Myanmar, just after 3am in late August 2017, after shouts from the village warned that government forces had arrived. They had heard tales of what the army had done to the Rohingya in recent months – killing unarmed civilians, mass rape, burning homes. Anwar, a farmer, wanted to get his family out as soon as possible. “I wanted to pack, but my son grabbed me and said, ‘Let’s just go – leave everything behind,’” Nur says. “I put some clothes in a bag but there was shooting and I dropped the bag.” Fourteen members of the family left together on foot.

Twelve are crammed into three rooms, with no doors. When it rains, thick mud sucks the shoes off their feet

Eighteen days later they arrived at the camp, 130 miles away, having crossed jungle, mountains and the Naf river on the Myanmar-Bangladesh border. “Because the military were everywhere we were afraid that if we went by road they would catch us, so we went over the mountains,” Nur says. “We just followed all the other people. Sometimes my son carried me, but when we came down the hills, he couldn’t help me because both of us would have fallen. So I slid down on my backside.” She arrived on Anwar’s shoulders.

Almost a year on Nur lives with her son and daughter-in-law and nine of their children in one shelter. Zaheda’s oldest daughter, Rojia, and her son Sultan live elsewhere on the camp, as do Nur’s four other children, who arrived separately.

The camp has evolved into a makeshift metropolis. It has roads, bridges, hospitals, but no official schools or paid jobs because the Bangladeshi government does not plan to integrate Rohingya refugees with the local population. The camp offers shelter from the Myanmar army – but also the risk of landslides and disease.

Scenes of the mega camp of Rohingya refugees in Cox’s Bazar, Bangladesh. Photograph: Abbie Trayler-Smith/Oxfam

Run by the UN Refugee Agency (UNHCR), the International Organisation for Migration and the government of Bangladesh, with the support of more than 100 NGOs, the camp is about a two-hour drive from Cox’s Bazar, a seaside town still popular with Bangladeshi tourists. Until a year ago there was nothing much in the vicinity: some villages, schools and small refugee camps for Rohingya who were here before the recent crisis, some displaced for over 20 years. But then came the flow of refugees into Bangladesh – thousands every day. The camps in Kutupalong and Balukhali doubled in size and have now expanded into one “megacamp”. There are dozens of other camps in the region, housing a further 300,000 refugees. While there has been talk of repatriation, negotiations have stalled. There are now more Rohingya here than in Myanmar; the population is the size of Birmingham – and their lives are in limbo.

The Begums’ shelter is bare of furniture; I sit with the family on a thin plastic mat. The room is hot, the camp noises are loud. Twelve are crammed into three rooms, with no doors to separate them. The flimsy roof is weighed down with bags of sand. When it rains, thick mud sucks the shoes off their feet; when it is hot, their clothes are drenched with sweat. But at least they feel part of a community – they know some of the other families sleeping beside them in the camp from their village back home.

Today the shelter is even busier than normal; it is the scene of intense marriage negotiations. Dil Mohammed, who used to live near Zaheda’s village in Myanmar, has two sons, Yaseen, 20, and Mohashan, 19, and has travelled across the camp with his wife to discuss a match with Gulsar and Abeda. “My sons are very good and handsome and…” Mohammed pauses for dramatic effect. “They went to school. They are literate.”

Zaheda is open to the suggestion – she has known the boys since they were very young. “They are good. Their manners are good,” she says. But the Begums can’t afford the 40,000 taka (£370) dowry. They had been quite well off in Myanmar. “We had 25 acres of land, 12 cows, six goats and lots of chickens,” Zaheda says. Now they have nothing.

Six-year-old Arofa and her best friend. Photograph: Abbie Trayler-Smith/Oxfam

“Whatever they give we will accept, willingly, happily,” Mohammed says. My interpreter is doubtful: she’s seen conversations like this before. “That is what he says, but I don’t think he will accept anything less than the full amount,” she says. Nothing is agreed today. Zaheda cooks Mohammed and his wife treats – noodles and lentils – before they head back to their shelter. “By God’s grace I will come back,” Mohammed says.

Abeda explains how difficult it is for an unmarried Rohingya woman to go outside without a burqa. Here, they stay hidden

Abeda sits quietly on the floor of the makeshift kitchen. She has long chestnut hair, and appears unruffled despite the heat. In Myanmar she studied Burmese and Arabic (at school until the age of 12, then at home) and liked to do embroidery. Gulsar is smiley and giggly as she peels potatoes. Abeda explains how difficult it is for an unmarried Rohingya woman to go outside without a burqa – “because men see us, unprotected”. At home they had the freedom of high fences and their own land. “We could grow vegetables and pick chillies.” Here, they stay hidden away. “I don’t really do anything. I just sit here. I wake up. I do chores. I pray five times a day and that’s it.” Of the many essentials NGOs have to supply – clean water, a sanitation system, food – burqas are high on the list.

Outside, meanwhile, the expansion and restructuring never seem to end. After the emergency phase came women-only spaces, makeshift cafes, barber shops, ramshackle market stalls where you can buy limes, potatoes, fish. Vegetables and herbs grow in every conceivable space. But there is a big problem. Because of the enormous, all-at-once influx, about 4,000 acres of forest near the Teknaf wildlife sanctuary and Himchari national park had to be cut down to build the camp. Many shelters are stacked on steep slopes of unstable earth.

Zaheda with youngest daughter Arofa. Photograph: Abbie Trayler-Smith/Oxfam

In June, the UNHCR warned that the Rohingya are “living on sandcastles”, with up to 200,000 refugees at risk from flooding and landslides during the monsoon season, which runs from June to September.

When I visit in July one person has died in a mudslide, and the monsoon rains have caused a new road built by the Bangladeshi government to crumble and crack. Oxfam has begun decommissioning and replacing nearly 500 toilets (including those built by other organisations) because they are leaning over dangerously, or at risk of flooding or sliding into ravines. About 15,000 refugees are being moved to a safer, flood-free area, while every day vehicles get stuck in the mud. In the mornings the road into the camp is a gridlock of NGO staff in their SUVs, overloaded sedans, tuk-tuks and water trucks. Residents tell me about other, unexpected fatalities: part of the camp sits on historic elephant migration corridors between Bangladesh and Myanmar, and 10 people have been trampled to death.

Then there are the infectious diseases: diphtheria (42 deaths in six months, according to the World Health Organisation, and 6,800 suspected cases) and acute diarrhoea (3,336 cases in July alone). I meet a young Rohingya woman who had decided to go barefoot rather than lose her shoes to the mud. The mud carried bad bacteria; she has an infected foot and is now limping.

The UN warned that the Rohingya are ‘living on sandcastles’, with 200,000 refugees at risk from flooding

The camp is poorly lit at night, and many NGO workers fear that the chaos has opened up new opportunities for sexual predators. Floriane Echegut, protection coordinator for Oxfam, works closely with the women in the camp. “There are no doors on the shelters. It’s dark, so people can go freely and cannot be recognised.” She says there have been reports of harassment, rape, sexual assault, human trafficking. “Of course when we go to the bathroom we are afraid,” Abeda says. The communal wash rooms are a short walk away. Zaheda worries, too. She always accompanies her daughters, especially at night.

The roots of the Rohingya crisis are centuries old. Evidence of the ethnic group in Rakhine state stretch back to the eighth century. The population was significantly boosted when the British took over what was then known as Burma in the 19th century and brought thousands of workers from elsewhere in the colonies to farm the land in Rakhine, which had some of the most fertile rice paddies in Asia. The Rohingya were always keen to be considered distinct from the other communities in a country that is home to 135 ethnic groups. They dreamed of an independent, Muslim state; many believed the British would give it to them in exchange for political support. When they sided with the British during the second world war, and fought against mostly Buddhist Burmese nationalists, the animosity between the communities became entrenched.

The vast refugee camp in Cox’s Bazar, Bangladesh. Photograph: Abbie Trayler-Smith/Oxfam

When Burma became independent in 1948, the country’s ethnic groups scrambled to find their places in the new nation. The Rohingya held on to their government posts (a reward from the British), and campaigned for the autonomous state they felt they had been promised. But the Burmese authorities rejected their request. In 1962 a military coup ushered in a new era of repressive nationalism and brutality. The Rohingya were seen as having benefited from colonial rule, and were declared illegal immigrants.

Nur Begum was born just after independence, but her family was far removed from the politics of Rangoon and the world beyond. She grew up as the second of 10 children in a remote village in Rakhine state. They had their own land to farm, their own cows to herd. They grew potatoes, beans, lentils, chillies, watermelon. “We had a beautiful life,” she says.

Nur went to a madrasa set up by her grandfather when she was eight, and married at 12. Amir Hussein was the only son of a wealthy farmer who lived in a nearby village. Their wedding was an ostentatious affair. Nur was decked out in gold bridal jewellery: a nose ring, a nose pin, an ear cuff and stud earrings, a necklace with 22 gold beads, and 16 bangles – eight on each wrist. “I was very little and it was very heavy,” she says, smiling at the memory. Anwar was born when she was 20; she went on to have two more sons and two daughters.

In 1978, the government launched Operation Nagamin, a campaign to identify illegal foreigners. “The military were doing background checks on us,” Nur says. “They were asking us lots of questions.” Human rights groups also cited unlawful killings, rape, arrests and the destruction of homes. The Begums – Nur, Amir and the three children they had at the time – fled to Bangladesh, where they stayed for nine months, before the Burmese government agreed to the community’s repatriation.

Rashida with her son Nurul. Her husband now lives with a woman he met on a previous trip to Bangladesh. Photograph: Abbie Trayler-Smith/Oxfam

In 1982 the government cemented its stance with the Burma Citizenship Law: anyone whose ancestors settled in Burma after 1823 (around the beginning of the British occupation) could not be a citizen. The repression intensified. Rohingya property was impounded. The military took land from the Begums. “We had to sell most of the jewellery to feed the children,” Nur says. Rohingya could no longer attend the best schools, or move freely – within Rakhine state, to other parts of the country, even abroad.

Men were forced into hard labour; Nur’s husband had to carry rice sacks to the military barracks. “Every time they came to get him he would run away. When they could not get him, they would take our biggest cow. If we wanted it back, we’d have to pay.” In 1991 Operation Pyi Thaya (Clean and Beautiful Nation) triggered another wave of persecution. Nur’s husband died two years later – weakened, she believes, by the hard labour.

Myanmar’s military junta officially disbanded in 2011, but the civilian-led government that replaced it continued to demonise the Rohingya, fuelled by the anti-Muslim prejudices of the country’s dominant ethnic group, the Bamars; nearly 90% of the country is Buddhist. The government (still dominated by the armed forces) and the majority of the Burmese people saw the Rohingya as a foreign group with a separatist agenda, driven by Islamism and funded from overseas. “They thought we were terrorists,” Nur says. The Begums were forbidden to leave their village without paying a fee. “There was a curfew between 6pm and 6am. If we were caught outside during those hours, we would be beaten,” says Zaheda. “We were not allowed to turn the lights on in the evening. We ate every meal in the dark.” Military vehicles became a local fixture and foreboding filled the air. Men started to go missing; Nur’s oldest granddaughter, Rojia, 25, last saw her husband when he went for a walk after lunch last August. “He went out to get some fresh air and they [the military] picked him up. I think he is dead,” Rojia says.

On 25 August 2017, Rohingya militants attacked army and police posts, killing 12 members of the security forces. The subsequent crackdown was brutal. Médecins Sans Frontières estimated that at least 6,700 Rohingya were killed in one month alone. Satellite images showed that more than 200 Rohingya villages were incinerated. In 2017, the UN High Commissioner for Human Rights called it a “textbook example of ethnic cleansing”. This week, the UN released a report describing the situation as “a catastrophe looming for decades”, the result of “severe systemic and institutionalised oppression from birth to death”. It called for the international criminal court to investigate the Myanmar army for genocide and crimes against humanity.

In every shelter, there are stories of personal horror. Abdu, 27, tells me about being shot while hiding in the hills

When the army came, the Begums had buried the last pieces of Nur’s bridal jewellery on their farm. “If the military got to know anyone had any gold they would take it away,” Nur says. They didn’t have time to dig it up before they fled. “We heard the army came with bulldozers.”

In every shelter, there are stories of personal horror. Abdu, 27, a farmer, tells me about being shot while hiding in the hills after the military torched his house. “The army started shooting randomly,” he says. A bullet went through his pectoral muscle, into his upper arm – and then “into my neighbour”. He doesn’t know if the wound was fatal. “We had to leave him.”

Another man, Sikandar, 27, cries as he tells me what happened to his family last year. He got up to go to the mosque and saw the army waiting in the bushes; the military had taken to arriving on foot rather than by truck, “so people don’t know they are coming”. He ran back into the house and woke his wife and two children. He grabbed his 16-month-old son; his wife was behind with their daughter. They firebombed the house and then shot at Sikandar. “The baby was in my arms. I dropped him. When I looked up, I saw they were throwing the baby into the fire.” He sobs. “I remember it every night when I try to sleep.”

The trauma in the camp is incalculable. And as time has gone on, many women have faced an additional problem: their husbands abandoning them. Nur’s youngest daughter, Rashida, 25, lives in the southern part of the camp with her four children, the oldest seven, the youngest just six months. Soon after they arrived 10 months ago, her husband, Jamir, a former shopkeeper, said he needed to leave the camp to look for a job. (Although the Bangladeshi government does not permit refugees to work, many do in the black economy.) Jamir disappeared. Rashida eventually learned that he’d gone to live with a woman he’d met on a previous trip to Bangladesh. He had married her, and they had a 10-year-old daughter. “When he calls me, he yells at me. He doesn’t want to be with me,” she says.

She has stopped thinking he will return. She no longer has the energy to fight. “I have to do all the work because I don’t have a choice,” she says. “When I get the rations I cannot carry them all here, so I have to sell some of the food and then hire someone to carry it for me.” One in six families on the camp are headed by single mothers whose husbands are missing or dead, according to Oxfam.

The refugee crisis, meanwhile, shows no sign of ending. In a speech on 21 August, Aung San Suu Kyi, Myanmar’s de facto leader, defended her government’s handling of the Rohingya. “We, who are living through the transition in Myanmar, view it differently than those who observe it from the outside and who will remain untouched by its outcome,” she said. She did not name the Rohingya, but said: “The danger of terrorist activities, which was the initial cause of events leading to the humanitarian crisis in Rakhine, remains real and present today.” She offered “deep sympathy and concern for all displaced persons, especially the women and children”. But she would not be drawn into setting a timetable for the crisis to be resolved. Potential resettlement sites have been mapped out in Myanmar since January, but human rights groups are concerned about the safety of returning Rohingya.

The Rohingya are stuck in no man’s land. Illegal foreigners in Bangladesh; illegal foreigners in Myanmar. But somehow daily life goes on. “What is the use of worrying about the future?” Rashida asks. “I have to take care of myself and my children.”

• Names have been changed. Sally Williams travelled with Oxfam. For more on its work supporting Rohingya refugees, go to oxfam.org.uk/rohingya

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