Jay Slater, a secluded jetty, and a yacht whose tracking beacon suddenly vanished: The most intriguing theory yet is put to the test by FRED KELLY. Read what he discovered...

At 600 metres high, it's easy to see why the jagged coastal cliffs of north-west Tenerife are known colloquially as 'the Wall of Hell'. These sheer basalt rock faces, which border the protected Rural de Teno park, are formidable.

The water is 30 metres deep at their base, where waves have, over millennia, chipped away at the volcanic surface to create a profusion of hidden caverns, caves and overhangs.

Could it be that almost four weeks ago, on June 17, the missing British teenager Jay Slater stood on top of these cliffs — dehydrated to the point of delirium — trapped between the near-impenetrable ravine of cacti he'd just walked through and a 600m drop into the Atlantic Ocean?

Officially known as Los Gigantes (The Giants), these vast precipices are just a couple of kilometres from the spot where Jay Slater's mobile phone last transmitted its location. 

Indeed, in my interview with Chris Pennington on Monday, the former British military reservist who has joined in the search for Slater speculated that he left the main road in the direction of the cliffs, in the belief that it was a shortcut to the coast, where he had been staying.

A pamphlet published by the local tourism board warns: 'There are no safe paths on the cliffs.' And it's easy to see why.

They are not only steep, but also rugged — littered with protruding rocks, brambles and lethal, hidden drops. The only way to traverse them 'is with ropes, harness and a helmet', local guide Tami told the Mail.

If Slater — the 19-year-old trainee bricklayer from Oswaldtwistle, Lancashire — did indeed find his way here, disoriented after fighting through the thickets, a small stumble could have led to a bloody and sudden death. We must hope that he never ended up on these perilous cliffs.

But if not the cliffs, where could he possibly have disappeared to? There is another theory to explain the mystery that has confounded all who have searched for him.

Could it be that Jay Slater hurled his phone into the thicket to avoid detection, before taking an entirely different and safer route down to the shore? Could he then have boarded a boat and left the island?

Those who support this explanation cite Slater's alleged theft of a Rolex watch from an Eastern European man on the night of his disappearance, and say that the young Briton then fled. And given that the huge drugs underworld on the island has been a line of inquiry, there is also speculation that he could have been taken against his will.

Social media has even gone so far as to suggest that Jay has been human-trafficked to Morocco, though a photo supposed to advance this theory is far from convincing.

But could Jay, for whatever reason, have left the island?

Local guide Tami informed the Mail that there is a well-trodden path from Masca — near the Airbnb Jay stayed in before setting off early in the morning to get back to the coast — which takes you down to a secluded beach: the perfect spot from which to board a small vessel.

This is a theory that has gained traction in recent days, following the news that a 14ft yacht named Maruba was recorded sailing past Masca beach around the time of Slater's disappearance. Bizarrely, the boat mysteriously then vanished from electronic charts — an unusual event that could be explained by the captain purposefully switching off his transponder to avoid detection, though the Mail is in no way suggesting that is what occurred in this case.

In order to test the feasibility of the new theory, I visited Masca beach earlier this week.

From Los Gigantes Port, just south of the cliffs, I secured passage on a local boat captained by a rotund man in a white polo, who wished to avoid being publicly named but was keen to resolve the mystery. 'You're looking for the British teenager?' he rasped in Spanish. 'I want to help.'

Travelling at 14 knots, the skipper rode the waves, his eyes fixed on the horizon, except for the occasional glance over at the cliffs on our right-hand side.

It was midday and touching 30C — a time for mad dogs and Englishmen. But with a brisk wind and a creeping sense of unease, it felt closer to single digits. After 15 minutes, we arrived at Masca beach.

There, the Mail found a sturdy jetty made from concrete breeze blocks. There were two tourist boats moored nearby with 20 or so people on each, who were peering at a pod of dolphins.

But in the early hours of a Monday morning, when Jay could have found himself down here, it would have been completely deserted.

It is immediately clear that if someone wanted to put something, or a person, on to a boat without being seen, Masca beach would be the place to do it.

The captain cut the engine and we bobbed along in silence. Drifting on the tide, we floated towards the pebbly beach.

According to tracking data logged by the Marine Traffic website, the Maruba moored near the Barranco Seco beach, a couple of miles short of Masca beach, at 7.10am on the morning of Slater's disappearance.

By 8.50am, when Jay's phone last 'pinged', the Maruba was recorded travelling past Masca beach.

Could the vessel have stopped there to pick up the teenager? And if so, was Slater embarking of his own free-will? The Belgian-registered vessel then performed an unexplained 360-degree loop before mysteriously vanishing from the electronic charts at around 10am.

Over the following nine days, the Maruba's movements remained unrecorded. And then suddenly, nine days later, the boat appeared again, for just two minutes, on the northern side of the tiny island of La Gomera, west of Tenerife and known for its wild, tropical rainforest.

The Maruba then started to head back towards Tenerife — recording its position again on June 28, a few kilometres off the Montana Roja nature reserve, near El Medano on the south coast.

From its direction, it was most likely either heading towards Gran Canaria or beyond to the African continent and Laayoune, the largest city in Western Sahara — or even Morocco, a five-hour sail away.

While the Mail is in no way suggesting the Maruba or its crew had anything to do with the disappearance of Jay Slater, or that the transponder was intentionally turned off, the coincidence of the yacht's location and the practice of 'going dark' by turning off its transponder — or automatic identification system (AIS) — is certainly curious.

AIS is mandated by the International Maritime Organisation and is designed to prevent all marine vessels from the risk of collision, particularly in poor weather or difficult sea conditions.

Turning off AIS is illegal except in circumstances where safety is at stake — in dangerous, pirate-ridden waters, for example.

Such behaviour is more often associated with drug smugglers, human traffickers and — in recent times — sanctioned Russian oligarchs seeking to hide their super yachts from authorities.

My captain pressed on the throttle once again and we continued up the coast to Juan Lopez bay. This is the spot where the ravine in which Slater's phone was last recorded comes to an end.

There were no boats in sight. 'It is illegal, we cannot stop here,' the captain pronounced.

Presumably, I thought, the police had closed the area off to resume their search for Slater, which officially ended almost two weeks ago on Sunday, June 30. Not a bit of it. 'From February to August,' he continued, 'no one can go within 250m of the coast, except at Masca beach. It's to protect the nesting sites of ospreys.'

Known locally as 'guincho', the number of these majestic birds of prey has plummeted over the past 20 years. A surge in visitors to the island, mooring in the bays and trampling across the birds' nesting sites has led to the ospreys being registered as an endangered species on the island.

A boat picking up a passenger such as Jay in these forbidden waters would have risked courting the attention of the authorities.

It would have been far safer to pick up the teenager at Masca beach. At this time of year, that is the only spot along the coast where boats are permitted.

But we cannot say definitively that Juan Lopez bay played no part in this story. Perhaps the absence of any other pleasure vessels in the area would have made this a more attractive spot to steal away a British teenager.

'All the beaches here are connected through a series of caves and tunnels,' Tami told the Mail. 'They are dark and difficult to pass, but you can walk from one to another if you know what you are doing.'

Could one feasibly survive in these caves for a short while? 'I could survive there for maybe a few weeks,' Tami continued, proudly. 'But then I know these cliffs. There are insects to eat and there is fresh water, if you know where to find it.'

Tami pointed towards a distant cliff, to show a water pipe that wraps around the rock. 'The pipe takes water from the town to the homesteads in the hills. You can get drinking water from there, if you're desperate. But if you don't know where the pipe is, you're unlikely to stumble across it.'

Tami's words raise the tantalising possibility that Slater — with local help — could have spent a number of days or nights hiding out here, possibly waiting for a boat other than the Maruba to steal him away.

Hiding down here, one would certainly not be chanced upon by the armies of amateur search groups that continue to scour the land up above without success.

Back in Los Gigantes, locals are reluctant to speak out about Slater's disappearance. It's clear that there is a code of silence here, and the town has closed ranks around its secrets.

'We're a real community,' Nat, a tourist shop owner, warns me as she closes up at the end of the day. 'If my daughter kisses a boy on the beach, I'll have someone on the phone telling me about it within a minute.' She laughs, lightly and briefly.

Then, with sudden solemnity and a look over her shoulder: 'It's probably time you headed home, don't you think?'

My route out of Los Gigantes takes me past the Catholic church, Parroquia del Espiritu Santo. On the inside of the door, there's a missing person poster bearing Jay's name and likeness. Beneath it, a rack of electronic candles lit by the faithful for a 50 cent fee.

As we approach one month since Jay went missing, it seems that whatever has happened to him, prayer might just be the only hope of bringing him home.

Additional reporting: Simon Trump