The drive to Treblinka is very green. I'm surprised. In my mind's eye, Poland has always been grey. My sense, informed by a Fantale-wrapper grasp of Polish history - melancholy Chopin; the Nazi invasion of September 1, 1939; the Russians, the Germans, then the Russians again - is of a pale, sad place.

But as I negotiate the narrow roads 80 kilometres from Warsaw, past fields and farms, through villages with unfamiliar names - Sterdyn, Kosow Lacki, Albinow - the only grey is in the lowering sky. Turning off the road into the secluded woods, the green is deeper still.

Only the slight double-bump as I cross the disused railway line into the Treblinka site disturbs this sylvan scene. This is the same line that brought the trains jammed with their human cargo to the death camp hidden here just over 70 years ago. It is the only part of what happened here that is still visible.

I am full of apprehension. This is my first visit to a Holocaust site. Only 24 hours earlier I had discovered that members of my family had been among the men, women and children shipped here like cattle, and I am unsure how I will react. As I step out from the car, what I cannot know is that I am about to have one of the most remarkable meetings of my life.

I am here as a subject of the television show Who Do You Think You Are?. For the last 10 days, the production team has been helping me time-travel in search of my father's line, originally named Ditkofsky, but changed to Denton in the 1930s because a Jewish name in East London, with the Blackshirts on the rise, was asking for trouble.

Skilfully, I have been shepherded from my home in Sydney, through London and Belarus, to the small village of Suchowola in Eastern Poland. It was from here that my great-grandfather, 27-year-old Israel Ditkofsky, escaped the jackboot of Russian rule in 1899 to make a new life for himself in England. And it is here that I learn how, in the autumn of 1942, Israel's stepmother, Masha, along with his half-brother, Eliezer, and his half-sisters Rachel, Gittel and Leah and their three children, became eight of the 5000 Jews force-marched, under whips and through snow, from the Suchowola ghetto to the trains destined for Treblinka.

Historian Jerzy Halbersztadt waits for me at the entrance to this silent place. He explains that there is little record of those who perished here at Treblinka because the killing was on such an industrial scale it was impossible to keep track of them. Of the estimated six million Jews killed in Europe during World War II, more than 800,000 perished at Treblinka - in just 16 months. It is one of the most murderous places on earth.

I ask Jerzy how he can be sure the Ditkofskys were among them. He describes a meticulous collection of German transport records showing train movements, dates and times, the number of people transported, all cross-checked against eyewitness accounts and court records. He then shows me a table with the numbers and dates of the people deported to Treblinka from the Bialystok region, where Suchowola is located. The dates and numbers square with the accounts from those Poles left behind.

Once off the train, he continues, they would have been first whipped and beaten, then driven through a gate into sheds and made to undress. The women's heads would be shaved. They would then be commanded to run, naked, into the gas chambers, to be asphyxiated by carbon monoxide from a diesel engine.

From the moment of their arrival to the moment of their death would have been - at most - three hours. Between Commandant Franz Stangl's morning coffee and his lunch each day, 6000 people died. After lunch, the killing resumed. Unlike the hundreds of work camps that were set up around Nazi-occupied Europe, Treblinka was built for one purpose only: swift mass murder.

The only Jews with any hope of survival were the tiny number picked to work as Sonderkommando - prisoners forced to perform unspeakable jobs: moving corpses, sorting through their belongings, taking gold from their teeth. Members of the Sonderkommando could hope to stay alive for a few extra weeks before they, too, would be executed.

In August 1943, the Sonderkommando of Treblinka staged a revolt, unheard of in the German camps, and 300 prisoners escaped. Three hundred out of the more than 800,000 sent to Treblinka. And of that 300, I was told, only one remained alive: 91-year-old Samuel Willenberg. "He lives in Israel," said Jerzy, "and he will be here to talk to you this afternoon."

Samuel Sillenberg, the last witness to Treblinka, is not the hunched, enfeebled survivor I had anticipated. He is a bull of a man, still powerfully built, his voice so strong that our sound recordist has to turn off his microphone and use mine alone to record our conversation.

No sooner have we greeted each other than Samuel takes off, abandoning his wheelchair and pushing through the bushes. Pausing to make sure he has got the spot exactly right, he jabs at the ground with his walking stick. "Here, here is where the fence was." Then, pointing again, "And that is where the guard post was."

Samuel has made it his life's work to remember and speak of what happened at Treblinka. He unrolls for me copies of his enormous sketches, drawn from memory, of the death camp. They are the only detailed record of the horror, because as the war turned against Germany, the Nazis destroyed all the evidence: Treblinka's buildings were levelled, the pits of dead bodies dug up and cremated, a farm created on the site.

Yet here Samuel has drawn the path along which the naked Jews were forced to run to the gas chambers. The Nazis called it Himmelstrasse - the Road to Heaven. Here is the Red Cross flag flying above the facade of a hospital to which the sick, the lame, and those too weak to walk were taken and then ordered to strip before being led to the pit, where an auxiliary guard shot them through the head. And here is the fake clock with painted-on hands that never moved, sitting on the fake railway station wall, to make Treblinka look like a normal work camp. This was a place where time, literally, stood still.

As a Sonderkommando member, Samuel would burrow through the possessions of the murdered - every day, a fresh mountain - searching for gold, jewellery, coins, and stock certificates to feed the Nazi war machine. Name tags, letters, family photos, anything that identified their former owners, were destroyed and the clothing shipped to Germany.

His voice rising with emotion, Samuel recalls the day, not long after arriving at Treblinka, when he was ordered to lift a bundle of clothes that had already been sorted. Among the pieces of clothing left strewn on the ground, he noticed a familiar colour - a small brown coat with sleeves lengthened by bits of green cloth. His mother had sewn those sleeves. They belonged to his little sister, Tamara. "A skirt worn by my older sister, Itta, clung to it - as if in a sisters' embrace."

I speak no Polish but such is Samuel's expressiveness, his intensity, I seem to know what he was saying even before the translator speaks. And he testifies not just in words. At the age of 70, having retired from his work as a surveyor, he entered university to study sculpture so he could document Treblinka with more than just a draughtsman's accuracy.

His sculptures, which can be found in Yad Vashem, Israel's official memorial to victims of the Holocaust, and the country's presidential residence, are a howl in metal. One is of a striking girl about 2 years old. Samuel had been ordered to shave her head: the Nazis needed the moisture-repelling qualities of human hair for their submarine crews' mattresses.

Often, Samuel would see a glimmer of hope in the eyes of the women whose heads he shaved. They believed the haircut was a prelude to disinfection; if they were being disinfected, they were to be kept alive. They did not know that, as soon as it was over, the SS officer would order them out - onto the Road to Heaven.

But this young girl knew. Samuel's words: "Though our acquaintance lasted only a few short minutes, I will never forget her. Her name was Ruth Dorfmann, and she had just finished her matriculation. Her beautiful eyes displayed neither fear nor agony of any kind, only pain and boundless sadness. She asked how long she would have to suffer. Only a few moments, I answered.

"A few minutes later, I heard the racket of the motor, which produced the gas. God must have been on holiday. I looked for Him, but there was only the beautiful Polish sky."

Hidden in the still pine forest, the memorial at Treblinka has 17,000 stones of different shapes and sizes, set in two broad arcs, as if a natural cemetery had somehow sprung from the earth. Seven hundred of them bear the names of Jewish villages in Poland obliterated by the Holocaust. One is engraved with the word "Suchowola".

As I drive from here all I can think is that every one of these little towns I've been passing through had been purged of Jews. Samuel had told me how, as the train slowed at stations on his journey to Treblinka, Polish people on the platform - sometimes children - would call out, "Jews! They're going to make you into soap!"

This man - wounded fighting the Russians when they invaded Poland in 1939; a fighter in the Warsaw uprising of 1944; a postwar leader of illegal trips across the Alps en route to Israel for Jewish children left in hiding by their parents; the last survivor of Treblinka - should never have outlived Hitler's Reich. But for two hours, through tears and with unwavering force, Samuel Willenberg has borne witness.

When we finish I hug him and promise that his story will be known in Australia, too. ■

Andrew Denton's episode of Who Do You Think You Are? is broadcast on SBS One on Tuesday at 7.30pm.