the first such use in the United States. Digging the Central Shaft also allowed workers to open two additional faces to excavate: once the shaft was completed in 1870, workers dug outwards from the center to meet the tunnels being dug from the east and west portals. Engineers built a elevator to hoist the excavated rock from the Central Shaft. One of the many engineering challenges posed by the project was getting the proper alignment between the four tunnel segments that were being dug: the east and west portal tunnels and the two tunnels dug outward from the central shaft. Engineers cleared a path through the forest over the mountain and strung a straight line from the east to west portals through "sighting posts" on the east and west peaks of Hoosac Mountain. In 1866, Thomas Doane took over as chief engineer. He resurveyed the alignment of the tunnel by constructing six towers. The alignment towers served to make sure the tunnel stayed true to its course. Grooved iron markers were originally used instead of these towers. Each tower, except the Rowe Neck tower, consisted of a transit scope, a sloped wooden roof on the top of the stone structure. Repeated surveys verified the line ran true between the posts, and steel bolts were installed at fixed intervals along the line. Only four of the towers remain today, in ruins, and can be found by using old roads and some bushwhacking through the current forest overgrowth. On December 12, 1872, workers opened the east portal tunnel to the Central Shaft-dug tunnel, which were aligned within : a tremendous engineering achievement at that time. On November 27, 1873, the remainder of the tunnel was opened to the west portal tunnel. The Hoosac Tunnel allowed the summit of the Fitchburg's route to be lower than the Boston and Albany, which only had one short tunnel. Lewis Cuyler of the Hoosac Tunnel Museum Society described the project as the "fountainhead of modern tunnel technology". The American Society of Civil Engineers made the tunnel an Historic Civil Engineering Landmark in 1975. Accidents and working conditions Deadly accidents during construction killed or seriously injured 195 workers with 135 verified deaths. Survivors dubbed the Tunnel the "Bloody Pit". Workers died from explosions, falling rock, and accidents involving ladders or scaffolding. In 1865, workers went on strike and burned buildings in protest. Fourteen men died or were