The bottom of the East River is sand - totally 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 the Roebling family, was easy. Its foundation is barely knee-deep in water, in contrast with the Brooklyn Bridge, which would be laid 40-80 ft. deep. The problem of the laying the foundation in such deep water was solved by using "caissons", massive wooden boxes (3000 tons each for the two Brooklyn Bridge towers), built air-tight, and floated down to where the foundation was to be dug. Stone was then layered on them until they sunk to the bottom of the river.
The water was 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, the bedrock was found at 45 feet but 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.