St. Peter's basilica dome studied by researchers

The Vatican says its researchers have probed the cupola of St. Peter's Basilica with high-tech tools, revealing 16th-century materials and construction techniques used to create the dome that dominate's Rome's skyline.
Vatican newspaper L'Osservatore Romano in a report Tuesday said research found that the 16th-century equivalent of today's reinforcement concrete was used to construct the dome, which was partially based on a design by Michelangelo.
The paper says one researcher climbed the 520-foot (136.5-meter) dome to use geo-radar to discover seven internal iron rings used to hold the travertine stone together.
Scholars, using centuries-old documents, had thought only two rings were used to girdle the structure. The Vatican says the dome seems to have been even more sturdily constructed than long believed.