16th Century dome of St. Peter's is strong

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.

Using high-tech instruments, the study found that the stone of the dome is supported by 7 iron rings, whereas it had been thought to have only 2 such rings. The dome, built in the 16th century, was constructed using the equivalent of today's reinforced concrete.

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.