To the east of Bottomside a hill rises abruptly to an elevation of 390 feet. Called "Malinta," it severs Bottomside from the tail end of the island. The North and South Shore roads curved from its slopes and girdled the hill and ran on down the curving tadpole tail. To facilitate passage and to create a bombproof shelter, the Americans drove a shaft from a rock quarry at Bottomside directly through the hill, creating the famous Malinta Tunnel. Construction of the tunnel took about 10 years. Work was initially started in 1922 and it was substantially completed in 1932. The 835 feet long East-West passage served as the main tunnel which is 24 feet wide and the height at the top of its arch is 18 feet. Branching from it are 13 laterals on its north side and another 11 laterals on the south side. Each lateral averaged 160 feet and 15 feet in length and width, respectively. A double-track electric trolley line used to run through the main tunnel which is reinforced with concrete walls, a concrete floor, and overhead arches. Blowers were installed to allow the circulation of fresh air.