As the stone towers were built on top of the caissons, the men beneath, dubbed "sand hogs," kept digging ever deeper. Eventually they reached solid bedrock, the digging stopped, and the caissons were filled with concrete, thus becoming the foundation for the bridge.
Today the Brooklyn caisson sits 44 feet below water. The caisson on the Manhattan side had to be dug deeper, and is 78 feet below water.
Work inside the caisson was exceedingly difficult. The atmosphere was always misty, and as the caisson work occurred before Edison perfected the electric light, the only illumination was provided by gas lamps, meaning the caissons were dimly lit.
The sand hogs had to pass through a series of air locks to enter the chamber where they worked, and the greatest danger was in coming up to the surface too quickly. Leaving the compressed air atmosphere could inflict a crippling ailment dubbed "caisson disease." Today we call it "the bends," a hazard to ocean divers who come to the surface too quickly and experience the debilitating condition of having nitrogen bubbles form in the bloodstream.
Washington Roebling often entered the caisson to supervise work, and one day in the spring of 1872 he came to the surface too quickly and was incapacitated. He recovered for a time, but the illness continued to afflict him, and by the end of 1872 he was no longer able to visit the site of the bridge.
For the next decade of construction, he remained in his house in Brooklyn Heights, observing the progress of the bridge through a telescope. His wife, Emily Roebling, trained herself as an engineer and would deliver her husband's messages to the bridge site every day.