St. Peter’s Basilica dome built to last … even better than earlier believed.

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.