One of the first things people instinctively do when a toddler goes missing in a holiday resort is check all the swimming pools. Every year, several hundred kids around the world accidentally drown in swimming pools when they wander off from parents.
This was possibly one of the first thoughts that preyed on the minds of Kate and Gerry McCann and the rapidly assembled search party that frantically went running through the streets and laneways of Praia da Luz in Portugal 10 years ago, on May 3, 2007.
Within minutes of discovering that the three-year old was missing from the ground-floor holiday apartment in the Mark Warner-owned Ocean Club resort pool, areas in neighbouring holiday complexes were checked in the vain hope that she might have wandered out of the apartment of her own accord.
Somewhat oblivious to what was unfolding on the streets below us, my wife and I and our daughter, who was only a few months older than Madeleine, went to bed in the Estrela da Luz apartment complex a couple of hundred yards from where Madeleine disappeared.
Having just returned from dinner in a nearby restaurant, we could hear commotion four floors below us on the street. It eventually moved on to another street and, thinking nothing of it, we headed for our wine-induced and, in hindsight, selfish slumber.
The following morning, our world would never be the same. But it would pale into insignificance when compared with the anguish of the McCann family.
A little English girl has gone missing overnight, we were told by one of the women working on the reception of the apartment complex. Hastily printed photos of a pretty, young blonde child had been left at hotel and apartment reception desks. This was the same Madeleine McCann we had seen tagging along with her mother as she pushed her twins in a double buggy up the hill. This was happening to the same family we had passed on several occasions days before as we walked down towards the supermarket. This was the same Gerry McCann we had seen playing tennis several days earlier.
The initial search the night before was called off around 4am, but as people woke up to the morning news, fresh search parties were immediately formed to scour the neighbouring hillsides and countryside while road-blocks were put in place by the police.
Later, a helicopter hovered over nearby fields and mountainous terrain, scouring every inch. A team of microlight gliders from a local club also joined the frantic search, focusing on areas difficult to access by foot.
Within 24 hours, the relatively quiet Hugo Beatty's bar and restaurant on the ground floor of the Estrela da Luz complex was transformed into a bustling press centre for the many journalists and camera crews dispatched to cover a story that had spread like wildfire.
In fairness, nothing had prepared the local police or indeed the regional police for anything like this. Not surprisingly, their initial unwillingness to share any information - possibly because they were quite literally clueless - was confused with ineptitude, a charge which stuck to them for much of the subsequent investigations.
Explaining to our daughter what was happening was difficult. Instinctively, parents want to protect children from all the horrific things that go on in this world. However, after we were asked to produce her passport at a police checkpoint just outside the village and officers examined her closely and looked at her passport several times to see if she resembled Madeleine in any way, we had to tell her the police were looking for a girl who had gone missing and that her mummy and daddy wanted her back. We told her they would soon find her. We all wanted to believe that.
Daytime searches gave way to night-time vigils at the church of Igreja de Santa Maria in the village. As Kate and Gerry McCann filed into the small church, the despair etched deep into their faces, onlookers lit candles. Others just wept.
Criminal investigators say the first few hours after an abduction are the most crucial. But as the days slipped by without any sightings or remains being found, a sense of foreboding gripped the village.
At no stage of the initial investigation, however, did local police attempt to interview us or anyone in our apartment complex, despite its proximity to the Ocean Club resort. In our case, certain things we had witnessed over the previous week and indeed on the night of Madeleine's disappearance might have been of use to them.
The suspicious looking characters hanging around Hugo Beatty's bar two nights earlier - one of whom resembled the identikit photo issued by the Metropolitan Police years later; the pick-up-style truck that sped past us as we walked back to the apartment from the restaurant that fateful night; the elderly bearded man videoing children at a nearby theme park three days before.
At the time, their possible significance meant nothing to us, but they might have been of some help. It was only when we arrived back in Dublin five days later that we felt compelled to volunteer statements to the gardai, who then passed them on to Interpol, by then already on the case. Presumably, Interpol then passed them on to the Portuguese police.
In addition, the many photos we had taken during our holiday that might have provided clues or identified 'people of interest' in and around Praia da Luz and Lagos were uploaded to a website that was set up by the investigation. We never heard anything back from the Portuguese police, who in July 2008 officially closed the case.
But it was a case that was never going to go away. With teams of private detectives, PR professionals and assorted high-profile benefactors like businessman Sir Philip Green and author JK Rowling, the Find Madeleine campaign never gave up hope, and a concerted effort was made to keep it in the headlines, something that irked the Portuguese police considerably.
This relentless campaigning eventually paid off when, in May 2011, Operation Grange was launched by the Metropolitan Police in London, following a request from the then Home Secretary, Theresa May, with the support of Prime Minister David Cameron. Assisted by the Portuguese authorities and a team of 29 detectives, Operation Grange initially sifted through more than 40,000 documents from Portuguese, UK and other European law enforcement agencies, as well as various private investigators who had worked on the case.
When all of this was complete, a full investigation was launched in July 2012, five years after Madeleine disappeared.
This time around, the investigation appeared to be a bit more professional, structured and detailed. The Portuguese investigation started on the back foot. Apart from the fact that they had missed that window of opportunity in the immediate aftermath of Madeleine's disappearance, accusations of leads not being followed up and sightings not being investigated were compounded by personal changes at the top of the team spearheading the investigation.
To many, particularly in the media, they were behaving more like the Keystone Cops than professional officers.
Of particular interest to the Metropolitan Police was the speeding pick-up truck that flew past us on the way home from the restaurant and the three suspicious looking in Hugo Beatty's bar. It later emerged that two waiters working in the restaurant also noticed a man acting suspiciously in a phone box across the road earlier in the evening. And one of the subsequent identikit photos published by the Metropolitan Police of two men they wanted to talk to resembled one of the men we'd spotted in the bar.
After carrying out a cell-dump of all mobile phone activity in the Praia da Luz area in the lead-up to Madeleine's disappearance and the days after, the Metropolitan Police were also able to identify all the telephone numbers we dialled, as well as the numbers that had dialled us during that period. As is the norm in such cases, details of all calls made and received were submitted as part of our statements.
The use of cell-dumps has helped police gain convictions in several high-profile murder cases in the past, and nowadays cell-dumps are almost as useful to the police as fingerprints or DNA. It was later reported by several UK newspapers that the cell-dump turned up a large number of telephone calls and texts in and around the Ocean Club between a group of men around the time of Madeleine's disappearance. Nothing ever came of it.
In total, Operation Grange took 1,338 witness statements, collected 1,027 exhibits and pursued some 560 lines of inquiry. In addition, more than 30 requests for help to other countries were made, while 60 people of interest were investigated. A total of 650 sex offenders, many of them paedophiles, were also considered, while reports of 8,685 different sightings of Madeleine around the world were investigated. These included Morocco, New Zealand, Malta, Belgium, France, Spain and Sweden.
Costing more than £11m, Operation Grange may have been wide-reaching, but so far it has yielded no results. Over the last 12 months, it has been wound down. Currently, a team of just four detectives is working on the case and pursuing, what former Metropolitan Police Commissioner Sir Bernard Hogan-Howe recently said was "one final lead".
For the McCanns, it might be too much to expect this final lead to yield anything meaningful. Ten years have passed and despite possible sightings, several key witness statements and a substantial amount of leads, the absence of any physical evidence leads one to the almost inevitable conclusion that the case may never be solved.
Over the last 10 years, numerous theories about what happened that night have emerged. The one that gained credibility among the media on the ground at the time was that she had been abducted by a paedophile ring and shipped out of the country that night, possibly on a yacht from nearby Lagos.
A number of sex offenders, including several convicted paedophiles of different nationalities, had been living in the Algarve and parts of lesser-known areas of western Spain at the time.
Several years before Madeleine's disappearance, a number of sexual assaults on young girls had taken place around the Praia da Luz area, but nobody was ever caught. Other iterations of this theory put it that she was possibly abducted by somebody with close knowledge of the Ocean Club and sold on to a group of gypsies that specialised in child trafficking and selling children to wealthy families overseas.
Several people, including Jane Tanner, one of the so-called Tapas Seven who dined with the McCann family, claim they saw a man carrying a girl in pyjamas that night. An Irish family, from Louth, also recall seeing a barefoot child fitting Madeleine's description being carried by a man in the town at 10pm, not long after it was first noted that she had gone missing.
Meanwhile, a taxi driver, Antonio Castela, says he picked up four adults and a girl resembling Madeleine the night after her disappearance and drove them to a hotel near Faro, where they got into a blue Jeep. Castela claims he contacted the police, but was never questioned.
In the 10 years that have elapsed since Madeleine's disappearance, however, only three official suspects have been named. The first of these named by Portuguese police was the British-born property consultant Robert Murat, who lived locally. Murat was the subject of a witch-hunt by several newspapers and was later cleared of suspicion, but not before several UK newspapers libelled him. He picked up around £600,000 in damages, while smaller settlements were awarded to some of his friends.
Then, in September 2007, Kate and Gerry McCann were also named as suspects, and the police formally put forward a case that Madeleine died in an accident and that they concealed her body and faked an abduction. This was the most controversial of all the theories and one that the former head of the investigation, Goncalo Amaral, reiterated in a book.
The McCanns later successfully sued Amaral for libel, and he was ordered to pay €600,000 in damages. Amaral's appeal against that decision succeeded in 2016, but the McCanns then appealed that decision to Portugal's Supreme Court, which ruled against them in February 2017. A subsequent appeal to the Supreme Court by the McCanns was also rejected in March of this year. Amaral has now indicated to the Portuguese Press that he intends to sue the McCanns.
As we near tomorrow's tenth anniversary of Madeleine's disappearance, it remains to be seen what approach, if any, the McCanns will take in the future. While they have reaffirmed their commitment to finding their daughter and have always proclaimed their innocence, their financial war-chest may be able to sustain their campaign for another few years. But it would also appear that they are running out of options. Unless new leads are identified, the Portuguese police may never re-open the case. As far as the Metropolitan Police are concerned, they have done their best and everything that was asked of them.
Meanwhile, back in Praia da Luz, it's business as usual. Locals are no longer willing to talk about the Madeleine McCann saga. They have moved on.
Popular opinion appears to be divided into two camps. In one camp sit those who believe that Kate and Gerry McCann were, in some way, complicit in their daughter's disappearance. Over the last 10 years, Facebook pages, Twitter accounts and websites have been used as platforms to spew all kinds of malicious accusations and theories, dreamt up by cranks, trolls and armchair detectives, all of which point the finger at the McCanns. But here's the thing: however rancorous these opinions are, they are just opinions. If the people in this camp have any evidence, then they know what they must do.
In the other camp, however, sit those who believe no parent could harm, either accidentally or intentionally, a helpless three-year old then try to cover their tracks. Many within this camp, including the McCanns, their extended families, friends and supporters, believe that Madeleine could well be living in some far-flung corner of the world, being reared by another family.
Sadly, there are some of us who believe that this story does not have a happy ending and that she is dead, possibly as a result of an unspeakable crime, and her remains may never be found.
But like all unsolved mysteries, we may never know.