Construction work on the station began in December 1933, and the engineers were faced with extremely difficult soil conditions from the outset. The area under Lubyanka square is made of Jurassic clay, beneath which are layers of quicksand and Carboniferous clay. The Metro station was planned to rest on top of the Carboniferous clay, which was thought to be firm enough to support its weight. It was quickly discovered, however, that the clay was much softer than anticipated due to the proximity of an underground channel of the Neglinnaya River and also tended to swell greatly when exposed to air. This meant that the tunnels had to be built one section at a time, very quickly, in order to allow the concrete to set before the pressure exerted by the expanding clay increased to the point where the wooden forms could no longer contain it.