Two rows of caissons have been excavated along the Nassau Street. The remains of the structural hollow tile floor arches can be seen at sidewalk level around the perimeter of the excavation. Lighter than the brick arch floor systems they replaced, the flat hollow structural tile arches used in skyscrapers at the turn of the century permitted the use of lighter steel sections and the construction of taller buildings for the same cost as the older, heavier brick arch construction they replaced.