The Brooklyn Bridge was by far the longest suspension bridge in the World. With a central span of 1600 feet, it was 60% longer than its nearest rival - the 1000 foot Cincinnati Bridge spanning the Ohio River. The length of the bridge wasn't the only issue either. It was the foundation also. By definition, the roadway of a suspension bridge is suspended by wires from towers above it, towers which therefore require an extremely solid foundation.

The bottom the East River is sand - unfit to support the necessary weight of the towers -and at the time no one knew how far down the sand went. The Cincinnati Bridge, also constructed by Roebling, was easy. Its foundation is barely knee-deep in water. The foundation of Brooklyn Bridge, in contrast, would be laid 40-80 feet deep.

The problem of the laying the foundation in such deep water still remained was solved by using caissons. Caissons were massive wooden boxes (3,000 tons each for the two Brooklyn Bridge towers), built air-tight, and floated down the river to where the foundation was to be dug. Stone was then layered on top of them until they sunk to the bottom.

Cut-away Diagram of a Caisson rising out of the East River

The water pumped out with compressed air, and workers could begin excavating the riverbed until they reached bedrock. Danger increased exponentially with each meter of depth. Fires, explosions and "caisson disease" - now known as nitrogen narcosis, or 'the Bends' - took the lives of 20 men, and left Chief Engineer Washington Roebling paralyzed.

On the Brooklyn side, they found it at 45 feet. On the Manhattan side, they never did hit bedrock. At 78 feet, Roebling decided that the weight of the tower would be enough to hold it in place. Only then did the men begin to build their way, brick by brick, back to the surface.