The Octagonal
An octagonal building is quite extraordinarily for the muslim architecture. For a long time it's purpose and date were completely unknown.
It's construction method and the material used indicated that it belonged to the same period as the other ruins in Samarra.
Ernst Herzfeld, a German archeologist who worked in Samarra in the early 20th century, had the theory that it was possibly a mausoleum. But the purpose of a mausoleum was completely unknown in the early muslim culture. He based his theory on the fact that caliph al Muntasir - who dies in Samarra in June 862 - had a Greek mother and it was delivered that she had erected a mausoleum for her son, even his two successors had been buried in his mausoleum. But there was no prove for his theory.
In December 1911 he was lucky to find three muslim tombs under the pavement of the Qubba and suddenly his former theory became sensational truth.
Why sensational? Because this building is first mausoluem ever built in Islam! and this was and is a sensation, indeed.