opening, while the floors of the openings are above mean water level, giving the towers a total height of above mean high water. Caissons The towers rest on underwater caissons made of southern yellow pine. Both caissons contain interior spaces that were used by construction workers. The Manhattan side's caisson is slightly larger, measuring and located below high water, while the Brooklyn side's caisson measures and is located below high water. The caissons were designed to hold at least the weight of the towers which would exert a pressure of when fully built, but the caissons were over-engineered for safety. During an accident on the Brooklyn side, when air pressure was lost and the partially-built towers dropped full-force down, the caisson sustained an estimated pressure of with only minor damage. Most of the timber used in the bridge's construction, including in the caissons, came from mills at Gascoigne Bluff on St. Simons Island, Georgia. The Brooklyn side's caisson, which was built first, originally had a height of and a ceiling composed of five layers of timber, each layer tall. Ten more layers of timber were later added atop the ceiling, and the entire caisson was wrapped in tin and wood for further protection against flooding. The thickness of the caisson's sides was at both the bottom and the top. The caisson had six chambers: two each for dredging, supply shafts, and airlocks. The caisson on the Manhattan side was slightly different because it had to be installed at a greater depth. To protect against the increased air pressure at that depth, the Manhattan caisson had 22 layers of timber on its roof, seven more than its Brooklyn counterpart had. The Manhattan caisson also had fifty -diameter pipes for sand removal, a fireproof iron-boilerplate interior, and different airlocks and communication systems. History Planning Proposals for a bridge between the then-separate cities of Brooklyn and New York had been suggested as early as 1800. At the time, the only travel between the two cities was by a number of ferry lines. Engineers presented various designs, such as chain or link bridges, though these were never built because of the difficulties of constructing a high enough fixed-span bridge across the extremely busy East River. There were also proposals for tunnels under the East River, but these were considered prohibitively expensive. The current Brooklyn Bridge was conceived by German immigrant John Augustus Roebling in