During the period from 1953 to 1957 the section connecting the western and southern underground railway systems of the City of Stockholm was completed. Ground conditions at the Central Railway Station, in the immediate vicinity of Stockholm's Strom (the body of water connecting Lake Malar and the Baltic), were such that it was elected to cast sections of the tube above ground in the form of caissons, which were subsequently sunk by means of special excavating devices to the desired depth. Three caissons were constructed, each about 35 metres in length and 19.5 metres in width. These were subsequently sunk to bed rock, about 14 metres below the surface of the ground. To prevent the entrance of sub-soil water, the work was carried out under excess pressure at a maximum of 1.3 to 1.4 atmospheres (atmos).