Tunnels of Gibraltar The tunnels of Gibraltar , constructed over the course of nearly 200 years principally by the British Army , have made the Rock of Gibraltar " a veritable warren of tunnels that housed guns , hangars , ammunition stores , barracks and hospitals " . Within a land area of only , Gibraltar has around of tunnels , nearly twice the length of its entire road network . The first tunnels , excavated in the late 18th century , served as communication passages between artillery positions and housed guns within embrasures cut into the North Face of the Rock . More tunnels were constructed in the 19th century to allow easier access to remote areas of Gibraltar and accommodate stores and reservoirs to deliver the water supply of Gibraltar . The 20th century saw by far the greatest extent of tunnelling when the Rock was turned into a huge underground fortress capable of accommodating 16 , 000 men along with all the supplies , ammunition and equipment needed to withstand a prolonged siege . The tunnelling finally ceased in 1968 when the British Army ' s last specialist tunnelling unit was disbanded . Since then , the tunnels have progressively been turned over to the civilian Government of Gibraltar , although a number are still owned by the Ministry of Defence and some have been sealed off entirely as they are now too dangerous to enter . Geology of Gibraltar . The Rock of Gibraltar is interpreted by geologists as a klippe of Jurassic dolomitic limestone with series of ' shale ' formations underlying and in part overlaying the limestone . It consists primarily of a series of carbonate deposits some thick , varying from dark grey bituminous dolomite at the base through to a very thick , apparently homogeneous sequence of light or medium grey fine - grained limestones . The formation appears to have been laid down in a tropical environment somewhat similar to the Bahamas today , and on the basis of fossil evidence an Early Jurassic ( Sinemurian ) age has been proposed for the Gibraltar Limestone , though in appearance it has a strong resemblance to the Carboniferous Limestone that underlies large parts of England and Wales . The formation ' s thick bedding , extensive cementation and dolomitisation have made it a generally stable engineering material . Tunnelling has been possible almost throughout the Rock , with the exception of one region where it has been precluded due to high ground water pressures . In some areas , zones of weak rock or intrusions of the underlying or overlying ' shales ' have caused tunnel instability . Phases of development . The tunnels of Gibraltar were constructed between 1782 and 1968 , in five distinct phases . 1782 – 99 . After the capture of Gibraltar in 1704 , the British began constructing fortified defensive lines on the north - west flank of the Rock of Gibraltar – the King ' s Lines , Queen ' s Lines and Prince ' s Lines . These consisted of trenches cut into the solid rock or natural ledges which were made more defensible by building masonry walls and scarping the cliffs below . The Lines were constructed in stages , building on earlier defences constructed by the Moors and Spanish ; the King ' s Lines reused Spanish defensive walls built prior to 1704 , the Prince ' s Lines were excavated between then and the Thirteenth Siege of Gibraltar in 1727 and the Queen ' s Lines were constructed during the Great Siege of Gibraltar ( 1779 – 83 ) . The first man - made tunnels in Gibraltar were created during the Great Siege when members of the British Army ' s Soldier Artificer Company dug out a tunnel behind the North Face of the Rock . It was intended to reach a position called the Notch , which was inaccessible by any other route , where the British garrison sought to install a new artillery battery to cover a blind angle . The tunnel was the initiative of the Company ' s Sergeant - Major , Henry Ince , who began the task on 25 May 1782 . As the works progressed , the tunnellers decided to create an opening in the cliff face to provide them with ventilation . It was immediately realised that this would offer an excellent firing position . By the end of the siege , the newly created Upper Gallery housed four guns , mounted on specially developed " depressing carriages " to allow them to fire downwards into Spanish positions on the isthmus to the north . The Notch was not reached until after the siege had ended ; instead of mounting a gun above it , the outcrop was hollowed out to create a broad firing position called St George ' s Hall . The tunnelling continued after the siege to build a series of connecting galleries and communication tunnels to link them together with the Lines . By the end of the 18th century , nearly of tunnels had been dug . These comprised : 1880 – 1915 . The second phase of tunnelling was driven by a number of disparate requirements towards the end of the 19th century as Gibraltar ' s naval base became a key strategic asset for the United Kingdom . Two short tunnels were constructed in Camp Bay on the southwest end of Gibraltar in 1880 to provide access to a quarry . In 1895 , the Army dug out two natural caves under Windmill Hill to create an underground ammunition store , the Beefsteak Magazine . Major new construction works in the harbour at the end of the century prompted the Admiralty to construct the Dockyard Tunnel between 1898 – 99 , driving through the entire width of the Rock in an east - west direction to reach Sandy Bay on the east side , where quarries were being worked to provide stone for the construction of the dry docks . In 1901 , the Admiralty followed up by extending the natural Ragged Staff Cave to form a tunnelled underground magazine adjoining the harbour . Gibraltar ' s long - running water supply problems were tackled between 1898 and 1900 with the construction of four underground reservoirs . They were supplied with rainwater gathered from catchment areas on the Great Sand Dune on the east side of the Rock and conveyed to the reservoirs by a new east – west tunnel . A fifth reservoir was constructed between 1911 – 15 . 1933 – 38 . The third phase of tunnelling was prompted by the rise of Nazi Germany and the outbreak of the Spanish Civil War . Air - raid shelter s and underground hospitals were constructed from 1936 , while improvements to Gibraltar ' s water supply were made between 1933 – 38 with the construction of four new underground reservoirs . An additional reservoir was begun but was used , in its incomplete state , as a storehouse and barracks for 4th Battalion The Black Watch during the Second World War . The improvements made during this phase increased the total length of the tunnels from to . 1939 – 45 . The most intensive phase of tunnelling in Gibraltar ' s history came during the Second World War , when the territory played a vital role in the North Atlantic and Mediterranean theatres . The total length of the tunnels increased greatly during the war , from to . At the start of the war , the civilian population was evacuated and the garrison was greatly increased in size . Numerous new tunnels were excavated to create accommodation for the expanded garrison and to store huge quantities of food , equipment and ammunition . The tunnelling was carried out by four specialised tunnelling companies from the Royal Engineers and the Canadian Army . A new Main Base Area was established in the south - eastern part of Gibraltar on the peninsula ' s Mediterranean coast , shielded from the potentially hostile Spanish mainland , and new connecting tunnels were created to link this with the established military bases on the west side . A pair of tunnels , the Great North Road and the Fosse Way , were excavated running nearly the full length of the Rock to interconnect the bulk of the wartime tunnels . The tunnels accommodated what amounted to an underground city . The entire 16 , 000 - strong garrison could be housed there along with enough food to last them for 16 months . Within the tunnels there were also an underground telephone exchange , a power generating station , a water distillation plant , a hospital , a bakery , ammunition magazines and a vehicle maintenance workshop . They also housed one of Gibraltar ' s most secret places – Stay Behind Cave , built for Operation Tracer , a plan to maintain a secret observation post manned by six men within the Rock if it had fallen to a German invasion . It was not rediscovered until as recently as 1997 . Among the major tunnels and systems built during the Second World War were : 1956 – 68 . The final phase of tunnelling took place during the Cold War to meet a number of new military and civilian requirements . Storage facilities were constructed under the Rock to hold bulk fuel , and two more reservoirs were also constructed . Several more connecting tunnels were excavated to improve road access across the south of Gibraltar . The final tunnel , Molesend Way , was completed by 1967 . Tunnelling ceased for good in April 1968 and the last Tunnelling Troop was disbanded and its members dispersed to other units . Excavation methods . Gibraltar ' s first tunnels were constructed by hand in a slow , backbreaking but remarkably long - lasting method of excavation . The first task was to fragment the limestone rock . This was done by using a variety of methods , including gunpowder blasting , fire setting ( building a fire against the face of the rock to heat it , then quenching it with cold water to cause it to shatter ) , quicklime ( used to fill boreholes which were then slaked with water , causing it to expand and so shattering the surrounding rock ) and hammering in wooden wedges which were expanded by soaking them with water , again causing the rock to shatter . The fragments were then removed with crowbars and sledgehammers . The tunnels thus created were about to in diameter with smooth - sided walls . Excavation was slow , at a rate of only about per year , but the methods used by the original tunnellers had the great advantage of causing only minimal damage to the surrounding rock . This has resulted in very stable tunnels which are still readily accessible today . The later tunnels were constructed at a far faster speed using explosives and machinery . By 1942 , the tunnelling had reached a peak rate of advance of of tunnel per week . However , the use of explosives produced large quantities of high - pressure gas which penetrated the surrounding rock and weakened it , and it appears that too much explosive may have been used . This has led to the surrounding rock fragmenting on a large scale , causing the Second World War tunnels to suffer from fracturing requiring them to be scaled periodically to clear away loose rock . Some of the tunnels have had to be sealed permanently as they are now too unsafe to enter . Others have needed to be supported by rock bolts , weld mesh , props , arches or tunnel linings , and their integrity is monitored by carrying out scanline surveys . The principal tunnelling method used during the war was the blast - hole diamond drilling technique , which had only recently been developed . It involved either undercutting the tunnel and bringing down the back using diamond drill blasting – detonating gelignite charges in drill holes spaced between to apart – or excavating the central portion of the tunnel to its full height and diamond drill blasting the sides . The resulting rubble was used to extend the airfield at RAF North Front out into the Bay of Gibraltar . Post - war tunnelling took place in less urgent circumstances and new , less damaging , tunnelling methods were used . Explosives were employed again but this time in smaller quantities , placed in drill holes bored into the face of the excavation . They were fired electrically in a sequence which began in the centre of the face and spread outwards , so that the centre would create a void into which the peripheral rock could fall . The downside of this method was that it created tunnels with jagged sides , as it fractured the limestone around the periphery of the excavation . Design and fitting out . Whereas the early tunnels were simple man - sized holes , the later ones – especially those excavated during the Second World War – were much more elaborate . The communication tunnels were initially dug out to a size of by in cross - section but were later expanded to by to allow small vehicles to pass . Many of the tunnels are large enough to accommodate vehicles up to the size of trucks . The main tunnels were dug out to a cross - section of by for through traffic routes , with a passing place every . The environmental conditions in the tunnels were unpleasant for those who had to endure them . The temperature was a steady to but humidity was up to 98 per cent , causing condensation and damp throughout . One of the most distinguished occupants was General Dwight D . Eisenhower , who used the tunnels as his headquarters for the invasion of North Africa – Operation Torch – in November 1942 . He later wrote : These conditions were alleviated by accommodating personnel in Nissen hut s and Iris hut s installed in chambers dug out to match their dimensions ( and spans respectively ) . The problem of rockfalls was alleviated to some extent by using arched soffit s in all tunnels of over in span . Some of the chambers were of a truly huge size ; the Royal Electrical and Mechanical Engineers ' vehicle maintenance workshop was housed in a giant chamber spanning with a height of and a length of . More conventional chambers generally had a span of to and a length of to , with a small section of tunnel called the " back drive " left open at the rear to connect each chamber to the next and facilitate the circulation of air . Without this , tunnels that were pure cul - de - sacs became unbearably damp . They were built in groups called " systems " which would each accommodate particular functions such as laundry , stores , accommodation and so on . The humid conditions in the tunnels meant that ordinary corrugated iron corroded extremely rapidly , so the buildings within the tunnels were constructed using iron sheeting that had been coated in bitumen . A gap of was left between the buildings and the rock walls to enable ventilation and drainage and to facilitate inspections of the rock . Power was provided by four electrical power stations with a total installed load of 1 , 200 kW ; water , electricity and oil were channelled via pipes and cables fixed to the tunnel walls , with drainage pipes running along the floors . The ventilation was provided by natural currents of air . During the Cold War , the tunnels were adapted to provide protection against blast and heat flash from nearby atomic detonations . Blast traps were added to both pedestrian and vehicle entrances . The pedestrian blast traps were relatively simple , consisting of three blast walls overlapping by at least three - quarters of the width of the tunnel . A more elaborate approach had to be adopted for vehicle tunnels , where L - shaped entrances equipped with blast pockets were cut . The blast pockets were intended to absorb and reflect the blast wave , preventing it reaching far inside the tunnel complex . Gas locks were also installed to protect against contamination from the outside . Tunnels today . The reduction in the British military presence from the 1980s onwards has led to decommissioning of many of the tunnels . The responsibility for maintaining them was devolved to the Government of Gibraltar ' s Ministry of Works and the then British Ministry of Public Building and Works , and their successors . Some of the tunnels are now open to the public . The Upper Galleries , excavated during the Great Siege , are now a tourist attraction under the name of the Great Siege Tunnels . The Middle Galleries and some of the Second World War tunnels can be visited and are advertised as the World War II Tunnels . The Lower Galleries are abandoned and theoretically off - limits . Most of the Second World War and post - war tunnels are closed to the public – some , notably AROW Street , have been sealed off altogether as being too dangerous to enter . The Admiralty Tunnel is now a secure data storage facility . Two of the post - war tunnels , Keightley Way and Dudley Ward Way , are open to civilian vehicle traffic to facilitate journeys between the west and south of Gibraltar , and along the precipitous east coast of the peninsula .