According to legend, pirate treasure reportedly worth £100 million is buried on an Indian Ocean island.

Although the region is thought to be littered with hidden treasure, this one is said to be the Holy Grail, the world’s biggest booty haul. The story, which reads like a Hollywood script, has been passed down through generations on the islands of the Seychelles and La Réunion.

Although many have tried – and failed – to locate the bounty, two men have devoted their lives to the quest. Reginald Herbert Cruise-Wilkins, known locally on the Seychelles island of Mahé as the ‘Treasure Man’, hunted the fortune for 27 years until his death in 1977. His son John inherited both the nickname and the quest.

When I first met John, he immediately barked that I was half an hour late. I didn’t expect a warm welcome; John is constantly hounded by writers and locals who stop him wherever he is, asking if he is looking for buried treasure.

This one is said to be the Holy Grail, the world’s biggest booty haul.

But as he showed me around what he believes is the treasure site, and talked about the clues and what he had left to do, the gruff man melted into one you couldn’t help but root for. His eyes twinkled and his smile was infectious. Even after all these years of searching, he was still the storybook boy hero armed with his backpack and treasure map, trying to piece together the puzzle. His is a story of hope and of never giving up, despite the odds.

John explained that the fascinating tale of the treasure started in 1716 when Frenchman Olivier Levasseur, otherwise known as ‘La Buse’ (The Buzzard) because of the speed with which he would attack his enemies, was given a letter of marque to operate as a privateer. But within a few months, Levasseur turned to the more lucrative career of pirating.

In 1721, Levasseur and his associates – then with 750 pirates over three ships – came across a Portuguese galleon flying British colours, Nossa Senhora do Cabo, in the port of La Réunion, then called Bourbon Island. They landed 250 men on board and killed the crew. Levasseur, who had no idea what was on the ship, was astonished with the haul. According to John, a historian described it as ‘a floating treasure house, believed to consist of gold and silver bars, precious stones, uncut diamonds, guineas, church plate and goblets.’

The pirates quickly fled to their headquarters in Madagascar with the British Navy in hot pursuit, and the booty was divided between the crew. “There was a share out; each pirate got 42 diamonds and 5,000 gold guineas a piece. There were extra shares for the officers,” John said.

Levasseur kept the rest.

The pirate then disappeared and is believed to have hidden his treasure on the Seychelles island of Mahé.

“He broke up his crew in groups of 20 men, my father thought. The crew didn’t know where the treasure chamber would be. [The treasure] was placed in a cave, kept for a temporary period, and then when the time came for the proper burial, it was only the burial crew who closed the cavern and they were then executed,” John said. No one except Levasseur now knew its location.

When Levasseur was finally captured and executed on 7 July 1730 on La Réunion, he apparently knew there were members of his pirate brotherhood in the crowd. He hurled a piece of parchment into the air, shouting 'My treasure for he who can understand’.

That piece of parchment was a treasure map in the form of a cryptogram of 17 lines.

“I can just visualise the hands going up, the fight, to meet the paper in the air,” John said wistfully.

But the cryptogram, which has been tested to be genuine parchment from the 18th-Century by the British Museum, just looks like 17 lines of jumbled symbols to the untrained eye.

For John, as for his father, the search has become his life.

Levasseur was an intellectual, a Greek and Latin scholar who was versed in masonic symbology. Cruise-Wilkins spent years trying to decode the cryptogram, using everything from Greek, Hebrew, astrology, astronomy, mythology and the occult to break the elaborate system of clues. Based on what he uncovered, he believed the bounty was buried according to a complex riddle inspired by the 12 Labours of Hercules, and, after many years identified the treasure to be in the stunningly beautiful area of Bel Ombre on Mahé’s northern coast, surrounded by turquoise sea, lush vegetation and huge smooth granite boulders sculpted by the waves.

Mahé is a small island where everyone knows each other. They all have an opinion on the story, from the taxi driver who drove me to Bel Ombre, to the lady who runs the guest house where I was staying. As soon as I told them what I was doing, they laughed, and reminisced on the legend they’d known since their childhoods.

“I don’t care what they think, but a lot of them think I’m crazy. They thought my father was crazy. And the question always with people is ‘Why do you treasure hunt? I mean, people don’t do this anymore’,” John said.

But for John, as for his father, the search has become his life, an intellectual and physical undertaking into which he’s poured all his energy and savings. When Cruise-Wilkins died in 1977, John was only 18. He grew up with the treasure hunt and it’s all he’s ever known. He wants to prove his father right, and like his dad, he wants to understand what made the pirate tick.

“I have found the mental treasure, it is only the physical that now remains,” he told me.

John has searched more than 40 acres around the Bel Ombre area, exploring crevices and caverns, and uncovering what he thinks are pirate markings on rocks. He’s also found bones, pistols, musket balls and statuettes.

I don’t care what [people] think, but a lot of them think I’m crazy.

“I’ve used all sorts of equipment: water pumps, rock drills, jackhammers, hand tools, even excavators and blasting at certain times, [as well as] the latest treasure detection equipment from Germany,” he said.

Although John is understandably vague on details, not wanting to be trumped in his search, he believes he’s found the actual treasure site, a cavern just off the beach – and mere steps from his home – which is blocked by boulders and can only be accessed by an underwater tunnel. John also warns that, according to his research, there will be a final booby trap over the treasure.

But it is illegal to dig up government land without a permit, and despite searching continuously since 1988, John was shut down by the government in 2009 until he can pay 250,000 rupees to get a license. He believes that this was done in order to secure the government’s stake on the booty haul.

“The government owns the land, so they are entitled to 50%, which is the law,” he said.

But still, 50% of £100 million is a lot of money, and John is emphatic that he won’t stop searching. Since 2009, he’s been doing preparatory work on the site, repairing rock armouring walls, building footpaths to get equipment in and ‘working on measurements, clues and other data to get this right’.

“After so many obstacles placed in our way had been overcome, the bond was the final straw. I almost gave up. But I could not give them that satisfaction,” John said. “My plans are to recover the treasure. To get a license. To get that bond removed, or else get the proper funding to finish off the work.”

Until that happens, the story of John and the pirates will never have a happy ending. And I’m rooting for him to have the last laugh.

Join over three million BBC Travel fans by liking us on Facebook, or follow us on Twitter and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter called "If You Only Read 6 Things This Week". A handpicked selection of stories from BBC Future, Earth, Culture, Capital and Travel, delivered to your inbox every Friday.