This was the British Pipe Line Under The Ocean undertaking by scientists, oil companies and the armed forces to construct undersea oil pipelines on the bed of the English Channel between the southern coast of England and the northern coast of France (1944/45).
The scheme was initially developed by A. C. Hartley, chief engineer of the Anglo-Iranian Oil Company, after the idea had been mooted by Vice Admiral the Lord Louis Mountbatten, head of the combined Operations command. As the Allied descent on the German-occupied continent of Europe was being planned and prepared, it was clear that their invasion and development forces would require a vast quantities of fuel, and the possibility of using pipelines was considered to relieve the dependence on oil tankers, which could be slowed by bad weather, were susceptible to U-boat attack, and were vitally needed for the Pacific campaign, where the distances and depths involved were too great for any pipeline.
Two types of pipeline were developed. The first of these was the flexible HIAS (Hartley-Anglo-Iranian-Siemens) type with a 3-in (76-mm) diameter load core, weighing some 55 tons per mile and in essence a development by Siemens Brothers, in collaboration with the National Physical Laboratory, of its existing under-sea telegraph cable. The second was the less flexible HAMEL (named for its two chief designers, H. A. Hammick and B. J. Ellis), which was a steel pipe of similar diameter. It was found in tests that the HAMEL pipe was best used with a final section of HAIS pipe at each end.
The first prototypes were tested in May 1942 across the Medway river in Kent, and then in June of the same year in deep water across the Firth of Clyde. The pipes then entered production, mainly in the UK but with some HAIS pipe manufactured in the USA. After full-scale testing of a 52-mile (83.5-km) HAIS pipe between Swansea in southern Wales and Watermouth in northern Cornwall, the first pipeline to France was laid on 12 August 1944 over the 80.5 miles (130 km) between Sandown on the Isle of Wight and Cherbourg, allowing the transfer of some 250,000 Imp gal (1.1365 million litres) of fuel every day. Another HAIS and two HAMEL pipelines followed. As the fighting moved closer to Germany 17 more pipelines (11 HAIS and six HAMEL) were laid from Dungeness in Kent to Ambleteuse in the Pas de Calais. The PLUTO pipelines were linked to pumping stations on the English coast housed in various inconspicuous buildings including cottages and garages. Though uninhabited, these were intended to cloak the real purpose of the buildings.
In England, the PLUTO pipelines were supplied by a 1,000-mile (1610-km) network of pipelines (constructed at night to prevent their detection by aerial reconnaissance) to transport fuel from ports including Bristol and Liverpool. In Europe, the pipelines were extended as the front line advanced, and eventually reached as far as the Rhine river. In total more than 172 million Imp gal (782 million litres) of fuel had been pumped to the Allied forces in Europe by VE-Day in May 1945, providing a critical supply of fuel until a more permanent arrangement was made, although the pipeline remained in operation for some time after that.