南京大虐殺・従軍慰安婦強制連行の嘘

A building shaped like a coffin is to shelter some of the victims¨ bones excavated from the "pits of thousands of bodies" right in the site when the memorial was in construction -- - an iron evidence for the bloody crimes committed by the aggressive Japanese troops.
-- - from the Official Web Site of the Nanjing Memorial

　 「大虐殺が有った」という方のどなたでも結構です。"an iron evidence" って何のことか教えていただけませんか？