The coastline of isolated and uninhabited Henderson Island was jammed with 18 tonnes of rubbish. An expedition spent two weeks collecting six tonnes of it to better understand why. Stuff's Andrea Vance and Iain McGregor were there.

With an easy leap, sailor Nikita Tkach plunged off the side of his ship into the deep, indigo ocean. 

As he scrambled into an inflatable raft, tethered to the enormous steel hull, passengers above him began shouting, laughing and pointing.

His disposable, yellow cigarette lighter had slipped from his pocket, and was floating on the water. Tkach, 27, dove beneath the waves again, swimming a few feet to retrieve the lighter between his teeth.

It would be the first piece of plastic recovered in a Herculean effort to rid Henderson Island, the remote Pacific atoll, of six tonnes of rubbish.

From Auckland, it takes six days to reach this tiny speck of land, deep in the Southern Pacific. After assembling in Papa'ete, the expedition group of three women and nine men left behind Tahitian sunsets and cocktails on a twin-prop plane bound for the Gambier archipelago.

The five-hour journey finished on a narrow airstrip on a surrounding motu. 

Nerves were stretched. En route, they'd learned the expedition's vessel, the Silver Supporter, had been swamped by bad weather, and its slow progress would delay arrival by a day. 

Laden with luggage, and cases of scientific equipment, they boarded the communal ferry to cross the lagoon to the village of Rikitea. 

A 19th century, twin-towered cathedral overlooks the turquoise waters of the tiny harbour. There was enough time to shop for local, black pearls while the island's agitated gendarme dealt with the immigration formalities.

Once the Silver Supporter slipped beyond the island's shelter, with the team onboard, she sailed south-east, towards British territorial waters and the Pitcairn Islands. 

Five days later, in the early hours of the morning, the engines finally cut. At first light, Henderson Island lay long, low and flat to the aft of the ship.

At sea, everything takes time. The inflatable raft was craned into the water, swinging perilously before dropping down onto the rolling swell. The crew flung a rope ladder down over the deck. It took 20 minutes to coax the raft's engine into life. 

For well over an hour, the expedition team lined the decks and watched impatiently as preparations for landing were made. 

It would be more than 24 hours before most of them made it onto the beach, after the craft capsized on its first run.

Henderson was colonised by Polynesians for around 300 years, until the 15th century.

In 1819, it was sighted from the British East India merchant ship Hercules and named for its captain, James Henderson. 

Since then, rough sea conditions, isolation and desolate soil have protected the atoll from human intrusion, apart from a few hardy scientists.  

Thirty years ago it was designated a Unesco World Heritage site, one of the best remaining examples of an elevated coral atoll ecosystem. As well as an important site for breeding seabirds, the island is home to four endemic land birds: a fruit dove, lorikeet, reed warbler and the plucky flightless crake.

In 2012, filmmaker Jon Slayer visited the island as part of an expedition to support efforts to create an enormous marine sanctuary in the island group.

The images he captured - of fishing nets and buoys, plastic water bottles, helmets, and crates scattered over more than two kilometres of beach - were uploaded to Google Earth.

An analysis, published in 2017, estimated 18 tonnes of plastic lay on the faraway shores and the findings appalled the world.

It was said to have "the highest density of plastic debris" recorded anywhere, with 3500 new items washing up each day. 

Two years later, Slayer, and the study's Canadian authors Jennifer Lavers and Alexander Bond, of the Adrift Lab, returned to the island with a dedicated team of volunteers.

The expedition, aboard the cargo ship that supplies Pitcairn Island, took more than 18 months of painstaking planning. It was scheduled for June 2018, but abandoned when the previous supply ship was taken out of service.

The British Government contracted a Norwegian shipping company, and in April, the Silver Supporter left Auckland with a Russian, Ukranian and Lithuanian crew. She was packed with enough supplies to last four months and containers full of science equipment, rubbish sacks, gloves, and scales to support the expedition.

Veteran British diplomat Robin Shackell, the deputy governor of the Pitcairn Islands, led the expedition. Slayer, a former Royal Marines Commando officer, warned him: "Battle plans rarely survive contact with the enemy."

With the shipwreck of five expedition members, Shackell's careful plans were set adrift.

The unpredictable surf, unforgiving reef, and long-range wind and swell forecasts meant he had to find an alternative route for the expedition team, before they could leave the ship.

Indefatigable conservationist Brett Howell was in charge of the beach clean-up. The lantern-jawed American bears more than a passing resemblance to Buzz Lightyear.  

His determination would push his crew - Shackell,  marine conservationist Johnny Briggs, recycling expert James Beard, and Pitcairn Islander Jay Warren - to the limits of their endurance.

Along with the ship’s captain, Sergei Kovalenko, Shackell, Warren and Howell drew up a plan. 

Warren determined he could slip the RIB (rigid inflatable boat) through a narrow, shallow channel on the north side of the reef, and land the team on a long strip of beach.

They'd be on the island, but it would mean a gruelling 5km walk through dense vegetation, and on craggy coral. Mid-morning, the team set off from the ship. 

Warren had visited the island many times before: more than two decades ago Pitcairners used to visit to harvest miro wood for carving curios to sell to passing tourists.

A path once existed, cutting through the narrowest point of the atoll. Warren knew roughly where it began, around 1.5km from the beach landing.

Between there stood three promontories, forcing the team to wade through water, sometimes more than waist deep. The coral underfoot was slippery, deeper trenches were filled with sea-cucumbers. The waves slapped off the headlands, unbalancing the walkers, who were carrying heavy loads.

Cold seawater filled their hiking boots, and rucksacks were lifted above head-height to keep them dry.

The path - marked long ago by a whitewashed piece of driftwood and a buoy - climbed steeply through the forest, then became a scramble up a 30-metre limestone cliff. 

On top, the route twisted around banyan trees and tangled scrub. Vines snagged their ankles and branches and sharp, coral spikes tore at bare skin. 

Briggs led the way, hacking away at the undergrowth. Behind him, Shackell, a trained mountain guide, kept him on a south-east path with a compass. He slipped on loose coral, leaving a large gash in his hand. By 2.30pm, he suggested they turn back, before high tide.

"I can hear the sea, just a few more minutes," Briggs replied. 

Within ten minutes, the group had broken out into the open, looking down over another steep cliff onto the beach below.

"We were very, very pleased to make it all the way across, because we weren't certain at that point, whether we had an expedition or not,” Shackell says.

The journey took an hour and 15 minutes. They would repeat it, twice, every day for the next two weeks. Most days they carried food, equipment, extra clothing and litres and litres of water. On one day, Howell lugged a car battery, Shackell following with 30kg of cement and Briggs with a solar panel to assemble a satellite camera on the beach.

Over time, the camera will send back an image a day, so that scientists can track the rate of accumulation of new debris.

Every morning, before six, Shackell was first up. He'd take a coffee mug out into the civil twilight and clamber up to the bridge to swap news and discuss the weather with Kovalenko.

The expedition settled into a steady, exhausting routine. Breakfast was cold eggs, porridge, sausage, cheese and toast. 

In the narrow galley, the team would shuffle around each other preparing coffee, making sandwiches for lunch, and deciding who would carry what.

At 7.30am the RIB launched, dropping the clean-up team and scientists on the beach. 

Underwater filmmakers Slayer and Luke Hosty, and oceanographer Simeon Archer-Rand would then take the craft out on the water, diving and mapping the seabed.

The beach clean-up started with Briggs measuring out a stretch of sand using his running app. The first task was to remove all the marker buoys from that area, throwing them into a large pile.  

On the first day, they would collect 300 buoys from a 110m stretch, buried into the sand and wedged into rock crevices. Some couldn't be retrieved, even using driftwood lumber to lever them out. In total, 1200 were gathered.

Then they moved onto the larger pieces of rigid plastic. Containers, water bottles, laundry hampers, and toilet seats were crammed into sacks. 

Anything larger than a bottle cap was picked up. The work was back-breaking, sweaty and laborious - a relentless task under the hot sun for up to six hours a day.

"It takes a lot of time on your hands and knees just shuffling through to grab it all," Howell says. "You certainly feel more impactful when you are going after the buoys and the big stuff because you really see the difference."

Alternate days were for science. Every item was counted, catalogued, weighed and recorded by Briggs and Beard so that Lavers and Bond can compare the data with that collected in 2015.

"The benefit of the in-depth data approach is that [the research] is publishable. But you sacrifice distance and the amount of material that you pick up because two team members have to be constantly processing data while the other team members are collecting," Howell says.

"On the first science day we picked up 350kg and on the off science day we picked up 940kg. So, massive difference and it just comes down to people power."

Beard, who limped through the fortnight with a sprained ankle, says the science days were the toughest. "Emptying all the bits of plastic that you have already collected out onto a tarpaulin and then counting each individual bit - it is an element of double handling. So, whilst the walk [over] is tough, it's the counting the small bits of plastic that I think everyone will be thrilled to see the back of."

Before the long trek back to the RIB, the team marked the end of each day with a photo. "It is so rewarding to see the progress you make as you move along the beach, you can see that there is a real difference in how the beach looks," Beard says.

The before and after snaps were shared, along with the collection totals, at a nightly team meeting. 

As the plastic piles accumulated, how to transport it all off the island was beginning to worry the team. The original plan was to use the inflatable rafts, but the sea conditions didn’t improve off the East Beach. 

The solutions became more fanciful. A blimp, an underwater tunnel, French nuclear testing? Someone suggested training the island's resident masked booby seabirds to fly it over. 

"Then we could call them plastic boobies," Slayer decided. 

The weather refused to settle for the rubbish to be removed during the course of the expedition. It now sits in 13 collection stations, dragged over the high-tide mark and piled up in beach scrub to be removed when it finally becomes practical to do so.

The evening always finished with a movie, cramped in the ship's lounge devouring ice-cream and soft drinks. The expedition worked its way through most of Jason Statham's film back catalogue.

Watching his action-man exploits, Briggs wondered how Statham would get the plastic off the beach.

"Statham wouldn't clean up the beach. He'd move the beach," Beard replied.