It was one of the most horrifying events in world history, resulting in the systematic murder of six million Jews as well as people from other groups considered by the Nazis to be inferior.
But while the Holocaust ended as the Second World War finished in 1945, the physical traces of that horrific period remain around Europe in gloomy abandoned sites that serve as a reminder of the genocide.
Now, photographer Marc Wilson has gone on a remarkable journey around 130 locations in 20 countries over six years to visit the areas and listen to the stories of survivors in English, French, Hebrew, Polish, Dutch and Russian.
He has collated 360 images for his new book, 'A Wounded Landscape: Bearing Witness To The Holocaust', which document some of the 40,000 sites that were occupied by the Nazis between 1939 and 1945.
Mr Wilson studied areas where Jews were killed along with the Roma, gay people, those with learning difficulties, the physically disabled, Jehovah's Witnesses, communists, and more than three million Soviet prisoners of war.
The remains of Ravensbrück in Germany, which was the second-largest concentration camp for women; the largest being the women's camp at Auschwitz-Birkenau. In January 1945, Ravensbrück had more than 50,000 prisoners from 30 countries
The disembarkation station in the valley below Natzweiler-Struthof concentration camp in Nazi-occupied France. There were about 50 subcamps in the local system - and by 1944, there were 7,000 prisoners in the main camp and 20,000 in subcamps
An area near the crematorium at Kulmhof extermination camp at Rzuchowski forest in Nazi-occupied Poland. The SS killed at least 172,000 people at the camp, also known as Chelmno, between December 1941 and March 1943 and in summer 1944
Prisoners from Mauthausen in Nazi-occupied Austria had to jump from this cliff to their death. It was known by camp guards as the 'parachute jump'. Some 197,000 prisoners passed through the Mauthausen system and at least 95,000 died there
Rita Weiss, was born in Romania, survived being sent to Auschwitz-Birkenau and Stutthof concentration camps. Speaking at her home in Tel Aviv, she said: 'I had to survive, I had to stay alive because I had to tell, must tell, what happened'
Radostowitz was a forest sub-camp of Auschwitz. Prisoners there worked felling trees, which were transported to Auschwitz II-Birkenau and used to burn bodies in the crematorium. The site, which was in Nazi-occupied Poland, is pictured in 2016
The roll call area and the remains of a barracks at Dachau concentration camp in Germany where more than 41,000 people were murdered. Dachau was the longest operating camp having been the first one established by the Nazis in March 1933
Bone fragments from burned and crushed corpses seep through the sandy soil at Kulmhof extermination camp at Rzuchowski forest in Nazi-occupied Poland, 2015. At least 172,000 prisoners were killed in the camp between 1941 and 1944. From 1942, bodies were burned in open-air ovens after the smell of decomposition from mass graves became apparent in nearby villages
He said: 'These sites persist today throughout these countries. Together they formed a pathway to genocide: destroyed communities and ghettos, internment camps, transit camps, labour camps, sub camps, concentration camps, extermination camps and displacement camps.
'They are connected by the landscapes that surround them, and the forced journeys made between them. At these sites, individual killings and slaughter on a mass scale took place, the numbers involved almost beyond our understanding.
'These are sites where literal life or death decisions were made, but they are also sites of hope, survival and memory.
Ben Barkow, former director of the Wiener Holocaust Library at the University of London, said: 'The Wiener Library has numerous opportunities to see artistic responses to the Holocaust, and I can state unambiguously that Marc's work is among the finest and most sensitive that we have seen in many years.'
The house of Amon Göth, the notoriously sadistic commandant of Plaszow concentration camp in Poland. The Austrian Nazi officer was also known as an adversary of Oskar Schindler, who shielded Jews during the Holocaust. He was hanged in 1946
Shmuel Atzmon-Wircer is a Holocaust survivor who lives in Tel Aviv. He and his family escaped to Russia during the war, but were sent to a forced labour camp in Siberia. He has devoted much of his later life to Yiddish theatre to preserve the language
The Krakow Ghetto. The 'liquidation' of the ghetto in March 1943 saw the SS kill around 2,000 Jews in the ghetto and transfer another 2,000 to the Płaszów concentration camp. Some 3,000 more were sent to Auschwitz-Birkenau, with most killed
A former ghetto site at Mucachevo in Ukraine, pictured in December 2018. On August 27 and 28 of 1941, many Jews of Mucachevo were murdered by the Nazis in Kamianets-Podilskyi's massacre. The remainder were deported to Auschwitz
Holocaust survivor Arthur Rose, in New York. He escaped with his sister Anna from the Ukranian ghetto in Lviv before it was liquidated, then went into hiding. The SS killed their parents, and the children reached Krakow on the eve of liberation
The Ancienne Gare de déportation de Bobigny. The station was used to transport French Jews imprisoned at Drancy to Auschwitz - meaning it was where many of them took their last steps on French soil. It is located on the outskirts of Paris
Grabnik square at Rivne, where 23,500 Jews were assembled and marched 2.5 miles (4km) to Sosenki forest to be killed on November 7, 1941. It is thought that ditches in the forest had been dug in advance of the massacre by Russian prisoners of war
The book has 736 pages and features a foreword by James Bulgin, who is the head of content at the Holocaust Galleries at Imperial War Museums.
The Nazis' concentration and extermination camps: The factories of death used to slaughter millions
Auschwitz-Birkenau, near the town of Oswiecim, in what was then occupied Poland
Auschwitz-Birkenau was a concentration and extermination camp used by the Nazis during World War Two.
The camp, which was located in Nazi-occupied Poland, was made up of three main sites.
Auschwitz I, the original concentration camp, Auschwitz II-Birkenau, a combined concentration and extermination camp and Auschwitz III–Monowitz, a labour camp, with a further 45 satellite sites.
Auschwitz was an extermination camp used by the Nazis in Poland to murder more than 1.1 million Jews
Birkenau became a major part of the Nazis' 'Final Solution', where they sought to rid Europe of Jews.
An estimated 1.3 million people were sent to Auschwitz-Birkenau, of whom at least 1.1 million died – around 90 percent of which were Jews.
Since 1947, it has operated as Auschwitz-Birkenau State Museum, which in 1979 was named a World Heritage Site by Unesco.
Treblinka, near a village of the same name, outside Warsaw in Nazi-occupied Poland
Unlike at other camps, where some Jews were assigned to forced labor before being killed, nearly all Jews brought to Treblinka were immediately gassed to death.
Only a select few - mostly young, strong men, were spared from immediate death and assigned to maintenance work instead.
The death toll at Treblinka was second only to Auschwitz. In just 15 months of operation - between July 1942 and October 1943 - between 700,000 and 900,000 Jews were murdered in its gas chambers.
Exterminations stopped at the camp after an uprising which saw around 200 prisoners escape. Around half of them were killed shortly afterwards, but 70 are known to have survived until the end of the war
Belzec, near the station of the same name in Nazi-occupied Poland
Belzec operated from March 1942 until the end of June 1943. It was built specifically as an extermination camp as part of Operation Reinhard.
Polish, German, Ukrainian and Austrian Jews were all killed there. In total, around 600,000 people were murdered.
Sobibor, near the village of the same name in Nazi-occupied Poland
Sobibor was named after its closest train station, at which Jews disembarked from extremely crowded carriages, unsure of their fate.
Jews from Poland, France, Germany, the Netherlands and the Soviet Union were killed in three gas chambers fed by the deadly fumes of a large petrol engine taken from a tank.
An estimated 200,000 people were killed in the camp. Some estimations put the figure at 250,000.
This would place Sobibor as the fourth worst extermination camp - in terms of number of deaths - after Belzec, Treblinka and Auschwitz.
The camp was located about 50 miles from the provincial Polish capital of Brest-on-the-Bug. Its official German name was SS-Sonderkommando Sobibor.
Prisoners launched a heroic escape on October 14 1943 in which 600 men, women and children succeeded in crossing the camp's perimeter fence.
Of those, only 50 managed to evade capture. It is unclear how many crossed into allied territory.
Chelmno (also known as Kulmhof), in Nazi-occupied Poland
Chelmno was the first of Nazi Germany's camps built specifically for extermination.
It operated from December 1941 until April 1943 and then again from June 1944 until January 1945.
Between 152,000 and 200,000 people, nearly all of whom were Jews, were killed there.
Majdanek (also known simply as Lublin), built on outskirts of city of Lublin in Nazi-occupied Poland
Majdanek was initially intended for forced labour but was converted into an extermination camp in 1942.
It had seven gas chambers as well as wooden gallows where some victims were hanged.
In total, it is believed that as many as 130,000 people were killed there.
Mr Wilson told MailOnline that he had 'wanted - or perhaps without realising it felt the need - to make a piece of work about the Holocaust for over 20 years now, ever since I started taking photographs'.
He continued: 'It is something I felt was of great importance, to talk about, to share, to start conversation about. But to be honest I simply never felt I could make the right photographs that a subject matter like this demands, or even therefore talk about it in the right way.
'I did not have the right voice or visual language. In 2015, after I had completed my previous book, The Last Stand, I felt, finally, that maybe I now did. A visual language that I hoped would be sensitive enough to talk about this tragedy.
'A quiet enough voice, but still insistent, to talk about a history that I felt does not need to be 'shouted' about but must not be ignored, forgotten or even worse, denied.'
He added that he had ideas about how he wanted to make the book and what he wanted to talk about, and took these with him to the first location he visited in the South East of France.
However, this all changed on his first late afternoon photoshoot, when he covered his head with a darkcloth and peered through the ground glass screen of my large format camera, which felt 'completely and utterly wrong to me', adding: 'I felt that I was gazing, objectively. And it was not right.'
Mr Wilson continued: 'My initial though was to photograph these locations from a distance, on the wider landscape around them, as I had done my previous work, but these images I was making were objective, wrong, cold, calculated.
'But I already knew what I had to do and had avoided thinking about. So the next morning I went over the former internment camp boundary, climbing over the mass artworks border that surrounded it and into the space itself.
'And there, after two hours of searching for the Barrack 'K12' the former children's barrack, I found my voice. The fading paintings on the walls, made by the children over 70 years ago, with paint provided by a Swiss red cross worker, cracked roof tiles on the ground and some small flowers in the surrounding grass.
'I was told what I needed to do. To tell the stories of those children. One of those children. And more. Stories of individuals who could then reflect the stories of a countless hundred others, 1,000 others, a million others.
'I made work there and back in the UK spoke with others, showed them what I was doing, had conversations and began to plan the time ahead.'
Asked what it felt like to visit the places, Mr Wilson said: 'Throughout making this work, over the six-year period, I was always very aware how lucky I was that I had this choice to visit these locations. I had the choice to make work there for a number of weeks and then return home.
'So I let them wash over me, both the locations themselves but also the details of events that I knew had happened there, that I had been told about, from someone who was there, during the war, in our conversations.
'Each place visited will stay with me but I don't think it is possible to successfully make work on this subject, this type of subject, from a distance.
'Today these places might be remembered or forgotten. Marked with a plaque or built over with a shopping centre, complete with McDonalds and multiplex.
'But each of the 150 plus locations visited is full of a history, full of individual stories, and in almost all of these places, full of horror and tragedy.'
He said that throughout making the book, he had been very conscious of not placing one location over another, and not giving one story more importance than another.
But Mr Wilson continued: 'Some places have left their mark on me perhaps more deeply than others. Either for the deep connection of a place to an event shared with me by one of the 22 survivors or family members or by what I saw, what that place made me feel.
'In some locations it was a faint and delicate shadow of a painting on a wall, drawn by a child's hand over 70 years ago and in another, at the at Kulmhof extermination camp in Rzuchowski forest, Poland, it was the human bone fragments seeping out of the sandy soil.
'Fragments of the murdered prisoners, whose bodies were burnt and crushed in an attempt to hide what had been done there. Each of these people, a murdered mother, father, child, grandparent, brother or sister, just like ourselves.'
He added that it took him 'some time' to find the courage to meet Holocaust survivors, saying he wanted to 'feel confident about what I could bring to their lives and thus them feel comfortable enough to share their lives with me'.
Mr Wilson continued: 'But each and every meeting, each one hour or four hour, or in some cases, two day conversation, left an indelible mark on me. Their kindness and compassion, their gentleness and their strength.'
And asked if there was anything he was told by one of the survivors said that particularly stood out to him, Mr Wilson said: 'I sat for days with these individuals, listening intently as they spoke to me in various languages, taking in each and every word, each glance, not asking questions looking for specific answers but simply listening.
'Hearing what they wanted to share with me, what they want the world to know. But if there is one sentence that I can share with you, it was spoken to me by Rita, in December 2017, aged 96.
An autopsy table in Buchenwald, which was was one of the biggest death camps established within Germany in 1937. From November 1938, more than 56,000 prisoners of the 280,000 kept at the camp were killed, including 8,000 Soviet soldiers
Woods beyond the perimeter fence of Natzweiler-Struthof concentration camp in Nazi-occupied France, about 30 miles south-west of Strasbourg. The camp, constructed in 1941, was enclosed by a 10ft (3m)-high electrified barbed wire fence
The view from a train window in Romania in 2018. Between 1940 and 1944, Romania allied with Germany, and conducted a series of violent persecutions or 'pogroms' against the Jews living there. The worst was in Iași, with 13,000 killed in June 1941
The Camp de Rivesalte internment camp, located near France's border with Spain, housed Jews of different nationalities as well as Catalan refugees and French gypsies. After the war, it was used as a prisoner of war camp for captured Germans
A photograph taken of the deportation route from Mukachevo in Ukraine, in what was then Munkacs in Czechoslovakia
A gas chamber at Auschwitz in Nazi-occupied Poland. The SS and police are estimated to have deported at least 1.3million people to the Auschwitz camp complex between 1940 and 1945. Of these, around 1.1 million people were murdered
'After she had told me the story of her childhood, her deportation from her village to an internment camp and onto a number slave labour camps, the murder of her family and finally the Death March from Stuthoff concentration camp in Northern Poland, and her group being left to float on a open barge in the Baltic Sea, with no food or water, left, on purpose, to die.'
Rita Weiss, whom Mr Wilson visited in Tel Aviv, Israel, told him: 'In April 1945 they ordered us out of the camp. There was no train or bus, we were on foot. The Death March.
'We came to the sea and there were barges. We waited to die in the sea. A barge with prisoners from Norway, Poland, Greece… After one day and one night we did not know what to do. One man said we must begin to swim.
'We did not know where we were, which country, which sea, we just wanted to swim so we did. We could die in the sea or we could die on the boat. I had to survive, I had to stay alive because I had to tell, must tell, what happened.'
'A Wounded Landscape – Bearing Witness To The Holocaust' by Marc Wilson is published by two&two press and available for £55 on the author's website by clicking here