In the final quarter of 19th century, the Great Western Railway (GWR) had undertaken the engineering feat of constructing the Severn Tunnel under the River Severn. One of the major difficulties encountered underground was the 'Great Spring', which necessitated the pumping of over 9 million gallons (41,000 m³) of water per day, at Sudbrook, from the western end of the tunnel, conveniently located only three miles (5 km) away from the proposed site at Caerwent. Even during the great drought of 1934 the lowest daily return was 9.1 million imperial gallons (41,000 m³). The GWR used about 1.5 million imperial gallons (6,800 m³) per day themselves, so there was always a guaranteed daily surplus of 7.5 million imperial gallons (34,000 m³).