Swimming across the English Channel is the equivalent of climbing Everest for endurance swimmers. It is iconic, it is gruelling, and it can be deadly.
Ten swimmers have died crossing the Channel, but British adventurer Lewis Pugh managed something even more dangerous.
Last month he set off from Land Ends to swim the length of the English Channel. The 350mile journey to Dover is the equivalent of crossing the Channel 16 times.
Lewis, 48, needed to be on his guard at all times. He has swum through the busiest shipping lane on the planet, through water teeming with jellyfish, whirlpools, and crashing waves.
But the biggest danger was hypothermia as the sea sucked the heat from his body.
Lewis was swimming wearing nothing more than a pair of Speedos, a swim cap, and goggles.
He broke the swim down into sessions of now more than two hours, to allow his body to rest and warm up.
He averaged five hours in the water each day, covering anyway between six and 12 miles per day depending on the tides and other obstacles.
He had to time the sessions carefully to ensure the strong tides are not pulling him backwards.
There were also days where the sea is been so rough and dangerous that Lewis has not been able to swim at all, pushing the whole expedition back.
The maritime lawyer risked his life to raise awareness about the destruction plastic pollution, global warming, and over-fishing are causing to our oceans.
Lewis said: “I had my first swimming lesson as a little boy in a place called Whitsand Bay near Plymouth. I remember going down to the beach and playing on this beautiful, golden sand.
“If you go down there today, at the high tide mark you will find pollution everywhere.
“It has been brought across the Atlantic Ocean by the current and dumped on our shores.
“So yes, this swim is dangerous, but we are all in danger if we don’t do something about this crisis.
“Nowhere is safe. I’ve seen plastic pollution at the North Pole and the Antarctic, on the surface of the sea and at the bottom.”
Lewis is no stranger to peril in his mission to save the oceans. Three years ago he swam 5km in a blizzard across the Antarctic, further south then anyone else has ever swum.
The temperature of the sea was below freezing, seven degrees colder than the water that killed hundreds of passengers when the Titanic sank. Any colder it would have turned to ice around him.
The waves that crashed over the side of his support boat froze in the air before shattering on deck.
Lewis only survived because his body has developed a super human ability, called anticipatory thermogenesis, where a rush of adrenaline prompted his body to burn extra calories and raise his core temperature to protect his vital organs when he plunged into the icy sea.
This unique ability has earned him the nickname ‘The Human Polar Bear’.
“Jumping in is a violent shock, like being whacked in the chest by Mike Tyson,” said Lewis.
“You can’t breathe, you’re gasping for air and you start swallowing water. If you panic you’re dead.
“The word cold loses all meaning. I suffered frostbite in the tips of my fingers, they were snow white. My whole body ached from head to toe.”
Lewis slapped his arms on the water as he swam to generate more heat, but that increased the risk of attracting killers whales and leopard seals.
Thankfully Lewis avoided these deadly predators, but within minutes he was charged by a curious sea lion.
He recalled: “The guy driving the safety boat screamed at me and leaned over the side. He grabbed my arm and yanked me furiously onto the boat.
“The sea lion was coming straight for me. If it had collided with me in those conditions, the consequences could have been disastrous. It was scary stuff.”
The risk paid off. After Lewis completed that swim a string of world leaders met him and agreed to extra protections for the Ross Sea in the Antarctic, the most pristine place left on the planet.
The new restrictions came into place this year, banning industrial fishing to protect species like the emperor penguin from extinction.
Lewis has also swum at the North Pole – where he risked being attacked by polar bears and walruses – and in a glacier lake two thirds of the way up Mount Everest.
He was even hospital with pollution poisoning while swimming the length of the River Thames, just like comedian David Walliams when he attempted the same feat.
But swimming the length of the channel was his most difficult and dangerous challenge yet.
Speaking before his swim Lewis said: “It’s a relief to finally swim in water where nothing wants to grab me and drag me other. Even training in South Africa there was the risk from Great White Sharks.
“But this is the most difficult swim I’ve ever undertaken, but a long way.
“My swims in the Arctic and Antarctic were short, fast bursts of up to a mile at a time. It was too cold to stay in the water any longer than that.
“This is swimming 350miles in cold, tidal water, day after day, week after week.
“And in the Antarctic you don’t have to watch out for jellyfish. A friend who goes swimming off Plymouth warned me there are enormous blooms of jellyfish everywhere.
“I will be swimming some of the legs at night, visibility will be so poor I could swim straight into them.
"If I swim into an enormous Portuguese Man of War or a Lions Mane Jellyfish the sting is strong enough to put me in hospital easily and could even cause an anaphylactic shock.”
Being struck by a ship in the darkness, especially if fog settles across the sea, is an even more deadly risk.
It is impossible to eliminate that danger completely, but Lewis was shadowed by a yacht with a special flag to let other ships know he is swimming up the Channel.
Lewis spent six months swimming nearly five miles each day in the cold, rough seas off South Africa and the Falkland Islands to prepare him for conditions in the Channel.
He has to eat 10,000 calories each day to prevent rapid weight loss, which would increase his risk of hypothermia as it will take 50 days to complete the swim in two hour sessions.
Each day Lewis stops to clean beaches along the route with Surfers Against Sewage (SAS) and meet politicians and dignitaries, including Prince Charles, to raise awareness.
He hopes it will have an equally inspiring impact as the world’s biggest beach clean up in Mumbai, India, which he visited to help several times.
That clean up is led entirely by volunteers and is still going 30 months after it began. It has shifted thousands of tonnes of plastic from Versova Beach, which lies just a few miles from the heart of glamorous Bollywood.
Previously the shallow salt water was an open sewer and dead dolphins and whales were washed up, their stomachs lined with plastic.
Now the beach is so clean rare ridley sea turtles returned to lay their eggs in the sand for the first time in 20 years.
Lewis said: “When I first went there, the plastic was piled to shoulder level. You could not see the sand, just piles of milk cartons, concrete sacks and flip flops.
“To see turtles coming back to that beach now is remarkable. It gives you hope that people are listening and are prepared to do something about it.
“The tide is turning. Two years ago no-one was talking about plastic pollution, programmes like Ocean Rescue and Blue Planet have changed that.
“But there is now so much plastic pollution in our oceans it is breaking down in micro plastics, billions of pieces too small to see. It is impossible to clean all that up.
“So we have to stop this at source and prevent plastic from getting into the rivers and the sea. We have to stop single use plastic.
"There is no place for plastic cups and straws in modern society. It harms marine life so badly. The UK has to become a world leader on this issue.
“The ocean currents move plastic around and much of it will land back on Britain’s shores. Or you could be eating fish caught in south east Asia, near the mouth of a heavily polluted river, and it has lots of micro plastics inside it. That will have an effect on your health.
“My goal is to ensure is to ensure 30 per cent of the world’s oceans are properly protected by 2030.”
You can follow Lewis' progress here - www.facebook.com/LewisPughOceans/