This week marked 70 years since the Auschwitz-Birkenau concentration camp was liberated by Soviet troops. To commemorate the anniversary, about 300 former Auschwitz prisoners returned to Birkenau's Gate of Death, the unloading ramp at the camp's rail entrance.

This aerial video of the concentration camp, taken by a drone, shows some of the most infamous locations within the confines, including the railway tracks into Auschwitz-Birkenau, the ruins of wooden huts at Birkenau, and the wrought-iron sign over the entrance bearing the words Arbeit Macht Frei - "Work sets you free".

The Auschwitz-Birkenau camp is now a museum run by the Polish Culture Ministry, and a Unesco world heritage site. Thousands of tourists visit every year, including survivors but as they age the number of living survivors is diminishing each year.

More than a million people died in Auschwitz, which was the largest camp established by the Germans during World War II.