Just look at that great, big, glorious dome, like a huge melon, divided into 16 sections, and held by 8 wonderful swinging arches on those extraordinary V-shape pillars and 28 columns through the church. It's like a vast net of stone and brick slung over this central space, this strange, mysterious space for the imperial communion.

 

It's a wonderful piece of architecture. And it's so... also it's a problem that you can't even see. You see those low domes exert tremendous pressure and there is a force in this building to push the bottom of it out, so the whole thing comes crashing down.

 

Now, Anthemius, like every other architect, has used stone here as lintels and beams, as stress and strain, the old way of doing things. He's come up with a brilliant idea to hold the church together, and it's this cornice. This huge beautiful marble cornice with inscription to Justinian and Theodora, this isn't just here for decoration, this links the church in a chain. It binds the stones together. A great necklace for the church brought from a shining island in a bright blue sea.

 

Throughout Justinian's long reign, the Marmara quarries were hard at work, shipping stone for a new crop of imperial churches. This was building on a grand scale, churches for every country in the empire. But the biggest of them all was a new church for the imperial communion in Constantinople. Through this, the quarry masters were cutting larger and yet larger versions of Anthemius's clever interlocking cornice.

 

Here is a piece of one of those stone chains under construction, and here's its secret. Each block was held to the next block by a great iron bracket held in lead run between the two stones. Anthemius's engineers used rather a lot of iron in their buildings. It's part of a whole new series of techniques that allow them to think more daringly, more bravely than any other architects have done before. Above all, it enabled Justinian himself to have the ambition to conceive of the greatest dome the world has ever seen.