Buried deep behind that imposing facade, regarded by the ancients as one of the Pillars of Hercules guarding the placid Mediterranean from the stormy Atlantic (and utilised by the British for just that purpose), is a bewildering and often beautiful network of natural caves and passages.
These are criss-crossed by a man-made warren - no less impressive in its way - of tunnels, roads, ventilation shafts, gun emplacements and escape routes spanning two centuries years of imperial history.
The methods used to penetrate the Rock were mostly rudimentary - a combination of painstaking hand-drilling and high explosives. But even though several generations of sappers have left behind 34 miles of tunnelling, much of it now unused, the "garrison in the dark" was constructed with such care that there is little likelihood of a landslip like the tunnel collapse that nearly engulfed Heathrow four years ago.
Although one of the tunnels is now a two-lane public road, providing a short cut from one residential part of the Rock to another, most of Gibraltar's 35,000 inhabitants are unaware of the sheer scale of the labyrinth beneath their feet.
It is a Herculean project that began during the four-year siege by the Spanish in the 1780s. Ever since those precarious times, the tunnels have been owned by the military; civilian access has been severely restricted.
But as the likelihood of air raid or nuclear alert recedes, various plans are afoot to open up the hidden secrets of the Rock to tourists and cavers.
The potential is enormous. "There are two things about Gibraltar which are unique," says Rock historian Richard Desouza: "The apes and the tunnel system. Leaving the tunnels hidden away is like going to a town and being shown one small shop, and ignoring an entire shopping mall down the road."
Indeed, far-off shopping centres spring to mind as the military signposts lead you from Queensway to Maida Vale via Clapham Junction, where a number of different systems converge.
During the war, four power stations provided electricity and kept the stultifying humidity at bay; reservoirs supplied each man with two gallons of fresh water and 25 gallons of salt water a day. It was here that General Eisenhower plotted the Allied invasion of North Africa.
Elsewhere, a hidden tunnel leads down a steep flight of steps from the cliffs to a secluded cove from which the governor of Gibraltar would have escaped had the Rock fallen.
It never did, of course. Today, with Gibraltar's strategic importance diminished, vast lengths of the tunnels are now disused and decaying: an important slice of history is in danger of being lost for ever.
The potential for converting the tunnels into a money-spinning tourist attraction is enormous. There's a comparable system on Jersey, where a solitary war-time tunnel, just one mile long, is visited by as many as 1.4 million fee-paying tourists a year.
Gibraltar looks on enviously. Keen to encourage tourism, but being short of development funds, it has to be content with its Barbary apes and down- market reputation as a haven for duty-free lager in British-style pubs.
As yet, only a fraction of its underground treasures are open to the public gaze. The best-known of these is St Michael's Cave, a massive fissure about 1,000 feet above sea level that gives on to a natural amphitheatre, is used today for concerts and light shows. It was once a fully equipped military hospital, complete with operating theatres, laundry and air-conditioning.
The typical visitor strolls gently through in 20 minutes, browses in the souvenir shop and returns to the sunshine and bustle of the town below.
A few stay on to don safety helmets, and join one of the unofficial tours deep into the spectacular Lower Cave, which was discovered during the war-time excavations.
Tito Vallejo, a senior guide who has explored every accessible inch of the tunnels and the 143 caves, delights in showing off their extraordinary features to anyone with a decent pair of trainers and a lack of claustrophobia.
Hidden lights illuminate a scene resembling one of those fantastical rock album covers of the Seventies - cathedral-like chambers, freshwater pools and lakes, spectacular formations of inorganic coral and limestone, giant stalagmites, translucent curtains of rock and ... utter silence.
Legend has it that an as-yet-undiscovered passage leads out of the cave and under the sea bed all the way to Africa. After all, where did those apes come from?
Tito is highly sceptical but, mindful of the Rock's declining fortunes, he would be delighted if a few thousand more visitors arrived every year to try to prove him wrong.
The territory's own airline, GB Airways, flies at least daily from Gatwick and weekly from Manchester, on behalf of British Airways (0345 222111).
The lowest fare for travel in September and October is a World Offer of pounds 192 return. Monarch Crown Service (01582 398333) flies on Mondays, Wednesdays and Fridays from Luton to Gibraltar.
Cave tours: Tito Vallejo (00 350 54244) takes parties down St Michael's Lower Cave for pounds 5 a head. Tours should be booked in advance, and last about three hours. Caving and expert scuba-diving parties by arrangement.
Gibraltar information: 0171-836 0777Reuse content