To reduce construction costs, the transport authority decided to use an abandoned canal bed to form part of the system, and did not dig deeply. By using the canal bed, the underground section is only 2–3 meters under the street surface, and was the world's shallowest metro. There were only 1,000 full-time construction workers at that time; the others were volunteers from factories, schools and institutions all over the city. The average number of volunteer workers exceeded 2,000 a day, which rendered the project unique in the world then. Construction was delayed in 1976 when an earthquake hit the city, but was resumed in 1979 and completed by 1984.