I remember the Philip Cairns case well. In fact, his disappearance was one of the greatest mysteries I encountered in my garda career.
It was a cold winter's evening in October when one of the biggest manhunts in the history of the state began.
Philip was a 13-year-old schoolboy at Colaiste Eanna in Rathfarnham and was on his lunch break at his home in Ballyroan Road.
At 1.30pm he left his house to make the 15-minute return walk to school, but he never arrived.
When his mother Alice came home from town, there was no sign of Philip. She rang the school to say that he hadn't returned. What started out as concern quickly turned to panic.
The headquarters of the gardai operation was in Rathfarnham and intensive searches were made of the local area. There was an individual house search within a mile radius of the scene.
Every attic and hiding hole was searched by teams of detectives and uniform gardai. Hundreds of volunteers combed the local golf clubs and the army and sub aqua teams searched every bog hole and river bank in the area. It was an absolutely massive undertaking.
I was Detective Sergeant in charge of suspects at the time.
A week into the investigation, an event that tipped the investigation and remains an intriguing mystery to this day was the discovery of Philip's school bag in a laneway off Anne Devlin Road, a few hundred metres from where he had disappeared.
A number of items were missing from the bag.
Strangely, on the night it was found it was raining heavily -- but the bag was bone dry.
It was one of these mysterious occurrences. Did the culprit bring it back and leave it at the scene as a red herring or was it something to taunt the investigators? Or could schoolchildren have panicked and left the bag back? This happened in pre-DNA times, but tests have since been carried out on the bag.
Philip's body was never found. Every sex offender and paedophile was interviewed in relation to an account of his movements between 1pm and 1.30pm on that day.
Despite the most intensive investigation, there was no sign of Philip. Suspects were eliminated.
The only other missing child is Mary Boyle in Donegal, who vanished in mysterious circumstances.
There have been up to 400 sightings of Philip. Each has been investigated, some across the water in Britain and in Europe, and many searches have been carried out in the intervening 23 years.
It has been a live ongoing investigation, but so far it has yielded no result.
The difficulty was that there was no crime scene, no body and no sightings of any suspicious activity. This makes it extremely difficult to bring to a successful conclusion.
This week's operation will raise hope, but we must exercise caution.
Throughout those years, his parents Alice and Philip and his family have held out hope.
I hope to God this time, without raising hopes too high, that this particular dig might yield evidence and give closure to the Cairns family.
There have been many false and hopes have been dashed.
While it gives hope, it is another harrowing experience for the Cairns family to go through.