When a child goes missing there is one thing worse than finding a body - and that's not finding a body.
Six-year-old Mary Boyle went missing from her grandparents' small dairy farm in a remote area of County Donegal in Ireland on 18 March 1977, the day after St Patrick's Day.
She'd helped her mother with the washing-up, played with her brother and sister and their cousins and then followed her Uncle Gerry as he set out across the boggy fields to return a ladder he'd borrowed from a neighbouring farm.
Uncle Gerry told the police that his niece had given up halfway along the path and turned back.
She was never seen again.
Mary had vanished without a trace. There wasn't so much as a scrap of fabric on a fence or hedge to give a clue as to where she'd gone or what had happened to her.
In our podcast, No Body Recovered, we set out to tell the story of the failed investigation into Mary's disappearance and how what happened on that March afternoon more than 40 years ago changed the lives of everyone who knew her.
We took our title from an entry in the log book of one of the police divers called in to make a fingertip search of the bottom of the lake beside the farm where Mary went missing.
Tosh Lavery recorded the wind and water conditions, the visibility, and the ominous words, "No Body Recovered".
Mary's disappearance was the main story in that week's edition of the local newspaper, the Donegal Democrat. A week to the day after she had vanished the paper said simply: "Her fate is a heart-rending mystery and any lingering hopes of finding her alive have now gone."
No-one would report an unsuccessful search for a missing child in quite such bleak terms now, of course, but, as we discovered, that's only one way in which the late 1970s seems like a vanished historical era.
Cashelard, the village where Mary went missing, is just a short distance from the border with Northern Ireland where the Troubles were raging with frightening intensity, occupying the attention of the media and draining the energies of police forces both in Northern Ireland and in the Republic.
Scientific investigation of crime was in its infancy.
It would be almost 10 years before anyone was convicted of a murder using DNA evidence - even if a body had been recovered, detectives would have faced a difficult task in gathering enough evidence to bring a prosecution. Without a body it must have seemed hopeless.
And this was a world without mobile telephones, without social media and without GPS - in those days very few homes in that part of Donegal would even have had landline telephones.
To report her missing, Mary's family had to attract the attention of a group of fishermen in a small boat on Lough Colmcille beside the farm and ask one of them to drive along narrow country roads in fading light to alert police in the nearby town of Ballyshannon.
We tracked down that fisherman - PJ Coughlan - and he helped us to recreate the story of those first few frantic hours.
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Someone knows what happened to Mary Boyle - but who? Kevin Connolly goes in search of answers, 42 years after her disappearance. Download the podcast No Body Recovered here.
"There was panic, surely," he told us. "With a wee girl missing on the mountain. They were all out roaring and shouting 'Mary, Mary' - we could hear them for maybe 10 minutes before we could see them."
PJ's first sight of Mary's mother, Ann, was of a desperate figure stumbling through the twilight on the steep, boggy ground that leads down to the lake. But like the other searchers who were to follow, Mrs Boyle could find no trace of what had happened to her daughter. Her memories of her last hours with Mary are heart-breakingly raw.
"On her last day," she said, "Mary said, 'Mam I didn't give you a kiss this morning' and she jumped up and threw her arms around me."
Shortly after PJ Coughlan reached the police station in Ballyshannon an appeal for volunteers to join the search was read out from the stage of the town's theatre which was staging its annual drama festival. With the podcast producer, Maria Byrne, we set out to tell the story of those desperate hours in No Body Recovered.
Getting people to talk about what happened to Mary wasn't always easy - the great Irish poet, Seamus Heaney, was born not too far from where Mary went missing. He captured something of the wariness of giving too much away to outsiders in the phrase, "Whatever you say, you say nothing."
But both Mrs Boyle and Mary's surviving twin sister, Ann, did talk to us - and our intention all along was not to follow the podcast genre of retracing an investigation, but to tell the story of how the lives of those left behind were overshadowed by Mary's disappearance.
In the case of the Boyles that is a tragic story. Mrs Boyle and her daughter Ann don't speak to each other because Ann believes she knows who murdered her sister and why. Mrs Boyle insists she doesn't know. The release of a YouTube documentary three years ago sharpened the division within the family.
Maria and I set out to tell both sides of the story. When you've heard both, you may be ready to decide who's right about what happened to Mary.
Stories of missing children have a unique power to haunt the imagination - think of Madeleine McCann and Ben Needham in England, or Etan Patz in the United States, the little boy in New York who was one of the first missing children to have his photograph posted on milk cartons as a form of state-wide alert.
Theories about what may have happened to Mary have circulated in Donegal for years, whispers that centre on the existence of paedophile rings in the north-west of Ireland. In the 1970s those rumours were passed on as gossip in pubs - on the internet, which is the modern equivalent, those whispers spread further and faster.
One such rumour - that Mary's disappearance might have been the work of the Scottish child-murderer, Robert Black - has proved extraordinarily persistent. We think we have finally disproved that theory, but we hope you'll listen and judge for yourselves.
The internet has made the fog of rumour harder than ever to pierce - no-one ever seems to come across a story online and find shades of grey, or uncertainty, or ambiguity in it. Everyone always feels they know, even when common sense should tell them that they only think they know.
As journalists we often deal in death as a statistic - the numbers killed in an air raid or an earthquake, or made homeless in a hurricane. It was humbling for us to deal so closely with the fallout from the story of a single missing person.
Somewhere, of course, someone does know what happened to Mary Boyle - it is a strange thought that if you read this or listen to No Body Recovered you may be reading and listening along with a witness whose memory may be jogged, or a perpetrator concealing guilty knowledge.
By a curious coincidence, the play on offer at the Ballyshannon Drama Festival the night Mary went missing was An Triail by the Irish playwright, Mairead Ni Ghrada.
It is a powerful piece of writing - I won't spoil the plot for you - but we were particularly struck by the words of a public prosecutor who appears during the trial itself.
"Dismiss from your minds," he tells the jury, "anything you may have heard or read about this case… I ask you to listen impartially to the evidence you are about to hear and to bring in your verdict accordingly."
That is our appeal to listeners to our podcast too, and it's a chilling thought that the volunteers who disappeared into the night at the beginning of the long search for Mary Boyle had that warning ringing in their ears.
The prosecutor's warning is a kind of antidote to the rumours and allegations and divisions that swirl around the case.
So remember his words, but above all remember Mary Boyle for whom the search goes on.
Correction 5th February 2020: An earlier version of this article wrongly referred to the child-murderer Robert Black as English and has since been amended to make clear that he was Scottish.