After the military uprising on 18 July 1936, the rebel troops advanced at great speed and Bizkaia came under threat. The recently-formed Basque Government then decided to create the largest infrastructure ever built in Bizakaia: a horseshoe-shaped defensive line, around 80 km long, consists of trenches, barbed wire, machine gun and mortar positions, strongpoints, shelters and refuges.
Its purpose was to protect Bilbao and its immediate surroundings, including strategic enclaves, such as El Abra harbour, the Lamiako and Sondika aerodromes and Bilbao’s main water (Zollo reservoir) and electricity (Burceña power station) supply infrastructures.
The work, which began in October 1936, were carried out by several thousands of people, most of whom were refugees and civilians, men under 18 and over 30, outside the military age, and women of all ages. In addition to the management, there were qualified staff, labourers, hauliers, guards, etc.