The towers rest on underwater caissons, giant upside-down boxes made of southern yellow pine. The Manhattan side's caisson is slightly larger, measuring 172 by 102 feet (52 by 31 m) and located 78. 5 feet (23. 9 m) below high water, while the Brooklyn side's caisson measures 168 by 102 feet (51 by 31 m) and is located 44. 5 feet (13. 6 m) below high water. The caissons were designed to hold 23 short tons per square foot (220 t/m2), even though the towers would only exert pressure of 5 short tons per square foot (49 t/m2) when built. The Brooklyn side's caisson, which was built first, originally had a height of 9. 5 feet (2. 9 m) with a ceiling composed of five layers of timber, each 1 foot (0. 30 m) 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. The thickness of the caisson's sides was 8 feet (2. 4 m) at the bottom and 8 feet (2. 4 m) at the top. The caisson had six chambers: two each for dredging, supply shafts, and airlocks. The Manhattan side's caisson was slightly different because it had to be installed at a lower depth. In addition to having seven additional layers of timber along with the 15 layers in its Brooklyn counterpart, the Manhattan caisson had fifty 4-inch (10 cm)-diameter pipes for sand removal, a fireproof iron-boilerplate interior, and different airlocks and communication systems.