Which emperor built the Pantheon's surviving form in Rome?

Hadrian, around AD 126 - and he left Agrippa's name on the front from the original building a century earlier, which has confused visitors ever since. Its dome was the largest in the world for over 1,300 years and is still the largest unreinforced concrete dome anywhere.

It stands because the recipe changes as it rises. Roman concrete in the base uses heavy basalt aggregate; higher up it switches to lighter tuff and then to pumice, so the dome gets progressively lighter towards the top, and the coffered ceiling removes weight again without losing strength. The oculus at the apex is open to the sky, which eliminates the heaviest part of the structure entirely - and rain simply falls in, onto a floor built with drainage.

Roman concrete also self-heals, which was worked out only recently. Lumps of unmixed lime in the matrix, long assumed to be sloppy work, turn out to dissolve when water reaches a crack and recrystallise to seal it - which is a substantial part of why a 1,900-year-old harbour mole is intact and modern reinforced concrete has a design life of a century. It survived because it was made a church in 609; almost everything comparable was quarried.