The Great Mosque of Samarra was, at its completion in 852 AD, the largest mosque in the world — a vast rectangular enclosure measuring 240 by 156 metres, with a roof supported by 464 pillars. Today most of the structure is gone, reduced by centuries of neglect and war to a field of low ruins, but the scale is still staggering. The thick outer walls survive to several metres in places, and the foundations of the prayer hall are clearly visible.
The mosque was commissioned by the Abbasid Caliph al-Mutawakkil as the centrepiece of his new capital at Samarra. It was used for less than a century before the capital reverted to Baghdad in 892 AD, after which the mosque fell gradually into ruin. What makes the site unmissable is not the ruins themselves but the Malwiya Minaret that rises beside them — the great spiral tower that has survived while everything else collapsed. The whole complex is a UNESCO World Heritage Site.