Planners and engineers began the process by acquiring the real estate, staging equipment and materials, and building a work camp. The 3,900 workers worked around the clock, seven days a week, to complete the project. Construction started by excavating the vertical shafts for all 20 tanks concurrently with mining of the upper access and lower access tunnels. The tunnels were aligned directly in the middle of the parallel rows of shafts. Once perpendicular to the shafts, cross tunnels were mined to connect the shafts to the main access tunnels. For constructability and safety reasons (cave-ins), the upper domes needed to be built first, so the miners excavated individual ring tunnels around the circumference of each of the future upper dome bases. They then scoped out the area of the domes and proceeded to construct the steel framing, steel liner and rebar. Workers continuously poured concrete that ranged in thickness from two feet at the crown to eight feet at the base. Once the concrete cured, it was pre-stressed by pressure grouting the area between the concrete and the basalt.