While some cold cases are being cracked through advances in modern technology, there's still a few that have left people baffled decades later.
One such case, known as the 'Lead Masks' case, involves the two Brazilian electronic technicians, Manoel Pereira da Cruz and Miguel José Viana.
While their deaths alone were never conclusively solved, it was how they were found that really stumped investigators.
For this, we're going to need to hark back to August 1966, when 18-year-old Jorge da Costa Alves stumbled upon them while out flying a kite at Vintém Hill in Niterói, Rio de Janeiro.
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A team of police and firefighters soon arrived - and what they found was bizarre.
Cruz and Viana's bodies were both dressed in formal suits with a waterproof coat and a lead mask which covered around their eyes - and they were partly covered in grass.
It only gets crazier.
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Beside their bodies, police discovered an empty water bottle and a bag with two wet towels and some money inside.
A small notebook was also uncovered which said: "16:30 be at the specified location. 18:30 ingest capsules, after the effect protect metals await signal mask."
Cryptic to say the least.
The pair were from Campos dos Goytacazes, a town to the northeast of Rio de Janeiro, which they'd left three days prior to their bodies being discovered.
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On the day they left, the two men had told their families that they were leaving to buy equipment and a car in San Paulo.
Having boarded a bus to Niterói, they arrived at 2.30pm and purchased waterproof coats from a local shop and a bottle of water from a bar.
During the investigation, police spoke to a waitress who described Migue as 'very nervous' and she noticed he would frequently check his watch.
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That was the last time they were seen alive, with the police concluding they went directly to the location from the bar.
But where do UFOs tie in to all this?
Well according to the conspiracy theories, it is said to have derived from a friend of the two men - who were both found with no obvious injuries.
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He claimed that the two men were 'scientific spiritualists' and were attempting to contact extraterrestrials or spirits using psychedelic drugs.
His theory behind them have lead masks was that it would protect their eyes against the possibility of them encountering a blinding light.
But instead of meeting aliens, they died of a drug overdose - or so the theory goes, as a toxicology report had deemed the bodies to be too decomposed to be tested.
While another theory also suggests that a UFO was seen where their bodies were found, according to a family who'd lived next to the area - and that another man had allegedly died four years earlier while wearing a lead mask.
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However, the case remains unsolved to this day.
Topics: Conspiracy Theories
Joe Yates