The Red Hill design called for 20 vertical storage tanks, each the height of a 25-story building and installed below 100 feet of hard rock cover. The tanks' dome-like ends were designed to withstand the pressure of many thousands of tons of oil. Two main working tunnels ran horizontally inside the ridge, with smaller side tunnels or "adits" branching to the tanks. As Woodbury described the design, "On paper the whole thing looked something like a fallen tree with two trunks, the vaults clinging in parts to the twin stems." Within each vault, workers contrived a steel shell as large as a house lot and backed by solid concrete, which abutted tight against the rock. As the concrete hardened, it shrank away from the steel. Workers then forced grout through pipes to fill every remaining crevice. Lili Reiki, Builders for Battle