Jean was married to John Thompson. He was a stevedore - working to load and unload cargo at the nearby docks. He was working on the fateful day, with no inkling as he set off for work that morning, it would be the last time he would ever see his wife.

As Jean and little Mark walked the short distance to visit her mother-in-law, she let her son run along in front of her - he knew the way and happily skipped ahead; keen to beat his mum to their destination. But just as he took a short cut along a flagstoned alleyway between Bill Street Road and West Street, to access the back of his grandmother's house, he heard an ominous rumbling noise.

As he turned, his mother had disappeared from view - with only a gaping hole in the ground left where she had been. He screamed.

His unsuspecting mother had fallen victim to a terrible twist of fate - tumbling into a huge chasm that opened up beneath her feet and would never give her up.

His screaming alerted the attention of postman Vance McCann who was on his round.

He said at the time: "I saw a little boy standing, petrified, looking at a hole in the ground and screaming. His grandmother came running out.

"I shouted down the hole but no-one replied and I saw no-one. I ran off to telephone the police and then went back to the alleyway to help keep people away."

Eric Thompson, Jean's brother-in-law, was with his mother when they heard the commotion.

He explained: "Mark came running into the house shouting 'It's Mummy, it's Mummy. She's gone'."

Emergency services rushed to the scene and police and fire crews spotted Jean lying in the hole - reportedly some 90ft below ground level.

Desperate attempts to reach her were made but no sooner had they spotted her prone body, she disappeared from view.

Firefighters were lowered down the hole on ropes, attached to the fire engine's ladder, in an effort to reach her.

But the fragility of the crumbling hole and fumes from the former cesspit which lay below - thought to have been broken in the collapse - hampered efforts. The rescuers struggled against falling rubble too - and two firemen were taken to hospital - one with chest injuries, another a broken arm.

A police officer at the scene told waiting reporters hours after she fell into the hole: "She cannot even be seen now and we believe she may have slipped into a tunnel or channel leading from the subsidence. There is 3ft of sand and water at the bottom of the shaft."

Safety concerns continued to hamper efforts the following day - while attempts to dig in the surrounding area in the hope of unearthing other entrances to the shaft also came to nothing.

Even efforts by the Royal Engineers' bomb disposal team, who were stationed nearby - with members lowered down the 4ft-wide hole in a cage - had to be abandoned due to the crumbling of the hole's walls.

Just 24-hours later, the hunt for her body was abandoned. It was believed her death, upon impact, would have been immediate.

Divisional fire officer George Muddle explained at the time: "There is now little chance of recovering the body. No doubt she drifted away into some underground working."

What is perhaps more likely is that her body was buried under the falling rubble caused by both the initial collapse, its instability following it, and the desperate rescue efforts.

Her body was never recovered.

Her distraught husband, John, understood the reasons for the rescue mission to be called off. He said at the time: "I accept the police and firemen had to stop work because of the risk to other people's lives.

"I want to see the authorities carry out a full investigation, so this does not happen to anyone else.”

Her tragic loss made headlines around the UK - including the front page of several national newspapers.

The situation was exacerbated and made all the more horrific by a decision - taken on safety grounds - to simply fill the hole in with the woman's dead body forever buried below. It took some five days for the gravel used to fill the gaping chasm.

So what could have caused the tragedy?

Speculation has ranged from an old underground cesspit collapsing to the roof of secret tunnels giving way. The truth is more likely to be found in the historical industry of the area.

A fascinating report into the land use over the years, published by the Kent Underground Research Group (KURG) in 1987 - an organisation affiliated with the Kent Archaeological Society - and republished 20 years later, shed light on the probe into the cause.

Chalk Mining and Associated Industries of Frindsbury, penned by Adrian Pearce and Dave Long, was prompted in part, at least, by the events of 1967 and a number of other holes which had suddenly emerged over the years in the area.

It revealed that immediately after the tragedy, Rochester-upon-Medway City Council - as it was known before merging with Gillingham Borough Council in 1998 to form the unitary authority it is today - commissioned a survey by engineering consultants WS Atkins & Partners.

Its findings, published in 1970, suggested a freak accident and that a general survey of the area was not warranted.

However, as more cases of subsidence occurred over the following years, a second report was produced, in 1985, according to the KURG report, "in which they stated there was a more significant danger to the public and property than had previously been implied".

So was the area's rich history in brickmaking and chalk mining responsible for the tragic loss of Jean Thompson?

The team from KURG, keen to understand the mines which may have lurked beneath the ground, initially set out to explore the underground network to find more evidence of a chalk mine in the area.

However, it immediately ran into a problem which may cause a shiver of concern for anyone now living in the area.

The authors explained: "All surface mining features had completely disappeared under a housing estate and any research had to be purely theoretical. Underground access would only be possible if a further subsidence occurred since all previous sites had been completely infilled prior to our interest.

"[In addition] mining records for the South East are sparse if not non-existent.

"As a result, we could not look at the mining history in isolation but had to include details of the associated industries, especially brickfields."

Worth noting, is that Frindsbury Brickfields once operated just a few hundred yards away from the collapse. And, of course, chalk is a key ingredient in the humble brick.

So, first, just how big was the hole into which the unfortunate Mrs Thompson tumbled on that fateful day?

With no official records taken, the KURG team was reliant on various accounts of eye-witnesses. It is believed the hole at the top was around 4ft wide. One key piece of information it did unearth was that a borehole was sunk into the hole two years after the tragedy to ensure the gravel poured in to fill it had settled.

Measurements determined the gravel went down 123ft. The hole itself stretched to 150ft before solid chalk was detected indicating the bottom. It is believed around 80ft of water filled the bottom of the pit.

The shaft itself was "obviously man-made" and had, according to the KURG report, been “capped over with a concrete slab”. It had then been converted into a cesspit for use by the nearby homes – a familiar set up in the days before mains drains became the norm.

At the time of the fateful collapse in 1967, it is believed it was still being used for the purpose and that eight sewer pipes and a road drain were emptying into it.

It is thought the corrosive nature of water and waste, over the course of many years, had “loosened the brick lining and caused most of it to fall away”.

People living in the area reported hearing rumblings in the days leading up to the tragedy, potentially caused by the ground around the shaft shifting and the concrete slab used to cap it off collapsing.

The report adds: “A shallow earth crust was left which collapsed under the weight of Mrs Thompson as she walked over it.”

Its final conclusion as to the shaft’s original purpose was that “there are really only two possibilities”. They were either a dene hole or a chalk mine.

Kent historian, speaker and guide Christoph Bull explained what a dene hole is.

He said: “There are lots and lots in north west Kent and every now and again they open up. And they can have catastrophic consequences - usually they don't - but they can do as a piece of path or road disappears.

“Dene holes usually go down deep and then fork off into various chambers so they look a bit like an upturned mushroom.”

Thought primarily to be used for mining chalk, there has remained much debate over the decades as to their original purpose.

Adds the KURG conclusion: “Whereas each [chalk mine or dene hole] has evidence in its favour, there are also factors against. There is insufficient evidence to point to the shaft being exclusively one or the other.

“There is, however, a third possibility which would answer the arguments against both features [and that is] it was originally a dene hole which was subsequently adapted for use by the Frindsbury Mine as a ventilation shaft.”

As Mr Bull reflects: “I'm 99.9% that poor lady was in the wrong place at the wrong time when a dene hole decided to make its presence known again.”

Reflects KURG: "The big problem is that nobody knows for certain how much potential there is for future subsidence.

"It is a certainty that there must be other old dene holes or chalk wells in the area.

"Apart from waiting for future subsidences to happen, not a pleasant choice, the only way is to carry out a programme of test drilling to discover and enter the workings.

"Once access has been obtained, an underground survey will reveal the extent of the problem. Such methods have been carried out elsewhere in Britain but they are very expensive.

"A further problem is going to be in deciding who pays for this treatment. Whereas the council is responsible for publicly-owned land, private land remains the responsibility of the landowner."

Medway Council has been approached for comment.

Read more: All the latest news from Medway