 Two caissons were sunk into the river bed below the stage. From there, the tunnels were constructed in each direction using Barlow-Greathead tunnelling shields of a similar design to those used to construct the C & SLR. The north tunnel was constructed first, commencing in February 1899, followed by the south tunnel from March 1900. This was technically the most difficult stage of the project, as it necessitated tunnelling under the river. The tunnellers worked in an atmosphere of compressed air at up to 35 psi (240 kPa) to prevent water leaking into the excavations. On several occasions, however, the tunnel was breached and escaping air caused "blowouts", producing water spouts up to 2.5 feet (0.76 m) high above the surface of the river. One such blowout disrupted Doggett's Coat and Badge race. By using the river as the centre of tunnelling operations, the company was able to remove excavated soil onto barges and bring in required material the same way, thus avoiding having to transport large amounts of material through the streets. Tunnelling also took place from station sites, notably at Piccadilly Circus. The tunnellers worked with a remarkable degree of accuracy given the technology of the time; the tunnel being driven north from the Thames eventually reached the one being dug south from Piccadilly Circus, meeting under Haymarket, with a deviation of only three-quarters of an inch (1.9 cm).
