Prior to the island's construction by the United States government, "Yerba Buena Shoals" of rock north of the transbay island had less than clearance and were a shipping hazard. The island was constructed by emplacing of quarried rock in the shoals for the island/causeway perimeter rock walls (a freshwater reservoir was quarried in the rock of Yerba Buena Island). Approximately of dredged bay sand filled the interior, was mitigated from salt, and then topsoil was used for planting 4,000 trees, 70,000 shrubs, and 700,000 flowering plants. Facility construction had begun by March 4, 1937, when two hangars were being built.