Just north of the Channel Tunnel lies an earlier tunnel. Unusually this was driven as a horizontal well, feeding a reservoir for local water supply. The tunnel is cut in chalk, above an impervious marl layer.
Originally built by the Folkestone Water Company around or just before 1900, it runs for around 500 metres into the North Downs. The exit point is at 220 feet above sea level and the adit runs upwards at an incline of around 1 in 150. Towards the end of the adit two side galleries run off, one to the east one one westwards. Two inch boreholes are driven up into the chalk around every 20 feet to increase the water catchment.
Civil Engineers surveyed the tunnel as part of the Channel Tunnel project, in order to assess conditions for the latter’s construction.The water tunnel was visited by Sub Brit in or around 1981 and its current condition is unknown.