The concrete dome is one of the wonders of ancient technology, an extremely complex and sophisticated design. The box structure gives it strength while reducing the weight. At the top is an opening, the oculus,which lets in light. (Also rain, which is carried away by nearly imperceptible drains.) The concrete dome was poured in place, probably using elaborate wooden scaffolding to hold it up while it dried.
have also shown that the concrete at the top was lightened as well, by using lighter stone in the aggregate:
On the lowest level travertine, the heaviest material was used, then a mixture of travertine and tufa, then tufa and brick, then all brick was used around the drum section of the dome, and finally pumice, the lightest and most porous of materials on the ceiling of the dome.One engineer calculated that these strategies reduced the weight of the dome by 40 percent. Hadrian's engineer was a genius, and as a result his creation is still standing after nearly 1900 years.
Ben Pentreath got up onto the roof with students from the American Academy in Rome
more pictures here.
It never ceases to amaze me that buildings as old as this one still stand in the world.