The Hartley Bottle Works were built by Thomas Delaval in 1763 and extended in 1777 making it the largest glass manufactory in the United Kingdom. At its largest extent it covered 12 acres and included workers houses, market place and a brewery. A central warehouse was connected to the glass houses by an underground narrow gauge railway, reputed to be the first in the country to run on iron rails and was designed by George Stephenson. Further tunnels connected the glass houses to the river banks for slag tipping. Another tunnel ran from the warehouse to the harbour to transport bottles to ships. The glassworks closed on 29th July 1871 following the decline of other industries in Seaton Sluice. The last cargo of bottles left on 20th May 1872 and the works were demolished in 1897. Some of the tunnels still survive below ground and were used as an air raid shelter during World War II (1939-1945).