Île Longue Île Longue ( French for " Long Island " ) is a peninsula of the roadstead of Brest in the department of Finistère in the Brittany region . It is the base of the SNLE , the French ballistic missile submarines , and as such one of the most secretive and heavily defended places in France . Geography . Île Longue is a peninsula on the commune of Crozon , attached to the Crozon peninsula . its altitude is 42 metres and has an area of 1 . 10 km2 . To the north of the peninsula there are two small islands : Ile des Morts and Ile Trébéron . Originally the isthmus was a mere sand bank , and crossing was only possible at low tide . During the 19th century , it was improved with a stone pathway . During the construction of the strategic base ( 1967 – 1972 ) , it was widened , made insubmersible , and changed into a big parking space . The peninsula is a stone plateau surrounded by cliffs . It harbours several water springs , which made human habitat possible . In the 19th century , three villages were built on the peninsula : Kernalleguen ( less than a kilometre of the isthmus ) , Kermeur ( 1 . 5 km of the isthmus ) and Bothuelc ' h , slightly North . History . Quarries . Exploitation of porphyry started in the 18th century . By the end of the 19th century , half a million cobblestones were produced each year . Up to eleven quarries were exploited around the peninsula . The largest , Vengleuz Braz , was located west from Kermeur . Fortifications . With its cliffs , local water resources , isthmus crossable only at low tide and its location defending Brest harbour , Île Longue is an easily defendable , strategic location . It was fortified by Vauban and Dajot as to render the roadstead impossible to hold by an enemy fleet and to allow a counter - attack against an enemy landing party attempting to seize Quélern . Dajot had a fort constructed at the top of the peninsula around 1776 . It featured a hemicircular lower battery with small buildings protected by a ditch , with a bastion and a drawbridge . In 1879 , a wall was built South of the peninsula , as to defend it against the land . It featured a drawbridge defended by a casemate and two bastions . Remains of the main gate and of the casemate still exist , but are enclosed in the military area , and not accessible to the general public . First World War . From 1914 to 1919 , Île Longue was used as an internment camp for German citizens captured after the seizure of the ship " Nieuw Amsterdam " , intercepted by the French 2nd Light Squadron on 2 September 1914 . From 3 – 23 September , the 1500 passengers were interned at Crozon fort and in Brest ; from 4 November , they were transferred to a camp built on Île Longue for the occasion . The last prisoners were released on 31 December 1919 . Second World War . During the occupation , the Germans used Île Longue as an anti - aircraft battery , which remained active until 1944 . Megalithic monuments were destroyed by the construction of bunkers . Île Longue strategic submarine base . In 1965 , President Charles de Gaulle chose Île Longue as home for the sea - based component of the Force de frappe . The place was chosen because it was close to a large harbour , yet distant enough to stay out of the public ' s eye , and was easily controlled and defended . Work began in 1967 and took 5 years . The peninsula was completely changed , as the isthmus was widened and consolidated , and the peninsula itself gained 30 hectares . Two dry docks were built , as well as numerous habitation , engineering and defence buildings . In 1972 , the base was made operational and " Le Redoutable " departed for her first patrol . Île Longue has been the base of the SNLE of the " Force Océanique Stratégique " ever since . Maintenance of the submarines is done at Île Longue , as well as storage of the nuclear components ( warheads and fuel bars ) . Chemical pyrotechnics for the missiles and conventional warheads are stored in Guenvénez , 4 km away . The base is divided into three colour - coded zones : Since 2006 , work has been undertaken at Île Longue to prepare the base for the new M51 SLBM . Controversy . In 2005 , a controversy broke out of this area is clearly visible from Google Earth . In fact , on this site it was possible to view satellite images of good quality to this delicate area . However , taking pictures from the surrounding coastline is banned by the authorities .