BBC News
When a walker called 999 to say he had seen a body in a wetsuit floating in a remote reservoir, detectives got to work.
Search teams combed the barren marshland on foot near to where he was found while a helicopter and drones searched for any clues on how and why he got there.
Investigators have asked UK police forces to check their missing people registers and have spoken to Interpol because the area can be popular with tourists.
Despite forensic tests, extensive inquiries and searches and appeals for information, detectives still have not solved the mystery of the man in the wetsuit.
He was found just before 08:30 on a Friday morning last October in the eerily still rolling mid-Wales countryside.
Investigators suspect he might have been in the vast Claerwen Reservoir for up to 12 weeks – so could have been in the water since as far back as July and in the height of summer.
What police know about ‘unusual’ death
What police say is “highly unusual” about the man in the wetsuit is that no evidence of his personal belongings or an abandoned car or bike were ever discovered.
But with no clothes or vehicle found and the nearest bus stop about four hours’ walk away, police do think it is unlikely he walked there in his wetsuit.
When emergency crews recovered the body, it was highly decomposed and taken to a nearby morgue for examination. Fingerprint and DNA tests failed to find any matches.
All police know is he is a white man, aged between 30 and 60, and was wearing a Zone 3 Agile wetsuit.
As the £200 wetsuit was extra large in size, officers expect the man was between 6ft and 6ft 5ins (1.83 to 1.96m) tall, weighed between 14 and 15 stone (89-95kg) with a 44–48in chest.
What’s the area like where the body was found?
It was a blustery February day when we travelled along the single track lane to the top of the reservoir’s imposing 183ft (56m) dam.
Mist covered the surrounding Powys landscape and there were no birds or other signs of life, beyond a few sheep dotted across the bare hills that belong to a remote farm above.
While we visited, a handful of visitors arrived in their cars to walk along the dam and back. We did not see a single walker or cyclist.
Even in the height of summer, when police think the man in the wetsuit might have died, locals say visitors to this particular reservoir are few and far between.
The nearby town of Rhayader is a cycling and walking hub and a popular stop-off for people driving between north and south Wales.
Alan Austin, who chairs a local rambling group and regularly walks in the area, said he has “never” seen anyone swimming in the water of Claerwen Reservoir.
“It can be extremely dangerous,” he said.
“The water can get very cold, water from the reservoir pours over the dams when it’s been raining.”
Other residents asked how someone could get there with no obvious means of transport, and why his belongings would not be found on the shore.
Rosemary Stow, who runs craft gallery Quillies, said most tourists do not make it past the visitor centre in the nearby Elan Valley.
“You don’t see a lot of cars up at Claerwen dam or in the car park below, it is such a lonely place,” she said.
“People here are asking, has he been dumped there? But if he was dumped why put him in a wetsuit?”
What do police believe happened to man in wetsuit?
The police’s working hypothesis is the man “entered the water voluntarily” sometime during the summer.
“It is not a known swimming area but that doesn’t mean it doesn’t happen,” Det Insp Anthea Ponting of Dyfed-Powys Police told BBC News.
She said her team were keeping an “open mind” on what might have happened, but said police can only base their theories on “known information”.
In July and August, the water temperature could have been 10C (50F) at the surface, but because of the reservoir’s depth it can become far colder.
Water under 16C (61F) is considered hazardous for cold water shock and signs along the shore warn against swimming or other water-based activities.
Claerwen Reservoir is the largest and most remote of the Elan Valley dams in Powys – and all five help provide Birmingham with its drinking water.
These dams are no stranger to deaths – 55 people die in inland waters across Wales every year and the local fire station has a boat for this reason.
Police have appealed to the public for help, but the tip offs they had have not given any new leads.
“It is an unusual situation to be three or four months down the line,” added Det Insp Ponting.
“In my experience there are not many missing people cases where we haven’t identified the person or identified loved ones after this period of time.
“At this time it is not believed to be any criminal activity. We always keep an open mind.
“The most important thing for this person is to know who they are and to ascertain how they died, and have that information for their loved ones.
“If anyone who hasn’t contacted us previously thinks they may know who this male is or have a concern for somebody and they feel they could be this person, please get in touch with us.”