Normally, in Nazi death camps, the ashes of the deceased at Auschwitz were hundreds if not thousands a day and were dispersed in rivers and in the surrounding woods.
The Nazis, under pressure from the imminent end (and defeat), began to disperse the ashes even in pits inside the camp that were then covered with water, especially in the last months of war. They were facilitated in this activity by the vast marshy areas of Birkenau.
The four black tombstones located a short distance from Crematorium 2, are dedicated to the ashes of the deceased, and are meant to remind us all of the atrocious way in which hundreds of thousands of prisoners were literally dissolved and dispersed in air and water.
In a way, visiting Auschwitz 2- Birkenau also means walking on lands still soaked in water and ashes, and making a real pilgrimage to the largest open-air cemetery, without-coffins, in the world.
SOURCES: