"When you looked at it with the naked eye, literally nothing was visible." Before and after images from the Manchester 1851 census
They were known as the 'Lost Souls' - thousands of people thought to have been lost forever from Manchester's 1851 census.
Their names were left virtually unreadable when the Manchester census reports were flood damaged in storage.
Now the latest forensic techniques have been used to decipher another 50,000 of the most difficult to read entries.
The work completes a task begun 20 years ago by the Manchester & Lancashire Family History Society.
When the 1851 census was first studied on microfiche, family historians from Manchester discovered that hundreds of thousands of entries were missing.
The reports of almost everyone who lived in the registration districts of Manchester, Salford, Oldham and Ashton-under-Lyne had been so badly damaged in a flood, they were considered too fragile to be filmed.
For those interested in discovering their past, it was particularly frustrating: the 1851 census was the first to include first names, full address and occupation and was a crucial piece in the genealogy jigsaw.
Back in 1991, Ray Hulley from Radcliffe was one of the first to handle the original 'unfilmed 1851 census' reports at the Public Records Office.
"Fortunately, the paper they'd used was quite strong," he said.
"The problem was that the records had been written in water soluble ink so, as soon as the water penetrated, it just washed the ink away."
For two days a week, he and other volunteers 'squinted' at what appeared to be blank pages to decipher what was left of the written entries.
When the records were moved to the National Archives at Kew, Ray continued his detective work, this time using ultraviolet light to read the names.
It was painstaking work. But after 14 years, they had recovered 178,000 names which they eventually passed on to the family history website findmypast.co.uk.
Now, researchers from another genealogy group, ancestry.co.uk have finally completed the task by deciphering the remaining 50,000 - and most difficult to read - entries and publishing them onto its website.
THE UNFILMED 1851 CENSUS
The 1851 Census was taken on the night of 30 March 1851
Census records include the full name, age, relationship, marital status and occupation of each person living at that address
217,717 names from Manchester - known as the Lost Souls - were considered 'lost'
Work by family historians and ancestry.co.uk has recovered nearly all the missing names
As Dan Jones, ancestry.co.uk's Director for International Content explained, the condition of the records left them with a massive challenge.
"The water damage was only the beginning," he said.
"It was the mould damage that was the worst so the documents were almost lace-like in terms of what was remaining.
"When you looked at it with the naked eye, literally nothing was visible," he added.
Led by Jack Reese, an expert in forensic photography, the team used a special multispectral imaging camera to view the documents.
It's a technique used in forensic science to investigate fraud cases, explained Dan, and also used to reveal the true colours of the Mona Lisa as shown in the da Vinci exhibition which came to Manchester in 2009.
But instead of a single clue to unravel, Jack's team had to work through 3,000 pages that were considered beyond repair and then index them for the website.
Slowly, the imaging technique teased out another 50,000 names from the past to add to the work done by Ray Hulley's team.
These included such notable Mancunians as parliamentary reformist Samuel Bamford; women's suffrage campaigner Lydia Becker and former Mayor of Salford, Edward Langworthy.
Sadly, some reports, particularly from the Pendleton and Pendlebury areas, have deteriorated so much, they may never be recovered.
Parliamentary reformer Samuel Bamford was one of the 'Lost Souls'
But Jack Reese said it had been a 'fascinating journey.'
"Forensic technology has enabled us to transform this collection of seemingly blank pages and scraps of paper into a hugely valuable set of historical records; and digitizing and transcribing them has preserved them for future generations," he added.
"The most exciting part will be when people start finding their 'lost' ancestors."
Ray Hulley, now aged 73, said it was obviously good news for people researching their family tree.
"We welcome the fact that, in the future, more people will be able to research their ancestors from the North West."
Free access to Ancestry Library Edition and findmypast is available at The Manchester Room@City Library or at any Manchester City Council library.