Bernini 's first work at St. Peter 's was to design the `` baldacchino '' which is like a tent or `` pavilion '' above the High Altar . This amazing thing is 30 metres ( 98 ft ) tall and is probably the largest piece of bronze in the world . Bernini 's first work at St. Peter 's was to design the baldacchino , a pavilion-like structure 30 m tall and claimed to be the largest piece of bronze in the world , which stands beneath the dome and above the altar .