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.