Deniers have said for years that physical evidence is lacking because they have seen no holes in the roof of the Birkenau gas chamber where the Zyklon was poured in. (In some of the gas chambers the Zyklon B was poured in through the roof, while in others it was thrown in through the windows.) The roof was dynamited at war's end, and today lies broken in pieces, but three of the four original holes were positively identified in a recent paper. Their location in the concrete matches with eyewitness testimony, aerial photos from 1944, and a ground photo from 1943. The physical evidence shows unmistakably that the Zyklon holes were cast into the concrete when the building was constructed.