When the Nazis left Treblinka in August 1943, they demolished the death camp, hoping to erase any trace of the 800,000 victims who perished there.

Only the stone memorial reminds the visitor of its terrible history.

"If you go in summer, it looks serene - butterflies flying around woodlands," said Dr Caroline Sturdy Colls. "If you don't read the information boards, you could well walk around the site and not have a clue what happened there."

But thanks to pioneering new research she has carried out, some of the secrets hidden beneath the soil have begun to emerge. The precise location of mass graves had not been known but the forensic archaeologist from Staffordshire University has now identified the site of 11 death pits.

"The biggest one was 34 metres long," she said. "The pits are so big that they are not something that could be produced naturally."

Mindful of religious sensitivies that prevent the disturbance of a Jewish burial ground, she and her team used a combination of scientific techniques such as ground-penetrating radar to probe beneath the earth.

Her investigations have also revealed evidence of buildings, including two solid structures in the area where gas-chambers were located according to eye-witness testimony.

Dr Sturdy Colls, who has previously helped police on cold cases, initiated the research, featured in this week's Radio 4 documentary The Hidden Graves of the Holocaust, for her PhD.

She entered the forensic field, she said, because "I feel strongly we should make sure that we record all types of crime. The Holocaust was the biggest atrocity that happened in Europe. People have got a right to know as much as possible about their relatives."