The outer wall is estimated to have required over 100,000 cubic metres (3,531,467 cubic feet) of travertine stone which were set without mortar; they were held together by 300 tons of iron clamps.[14] However, it has suffered extensive damage over the centuries, with large segments having collapsed following earthquakes. The north side of the perimeter wall is still standing; the distinctive triangular brick wedges at each end are modern additions, having been constructed in the early 19th century to shore up the wall. The remainder of the present-day exterior of the Colosseum is in fact the original interior wall.