Lower St Michael's Cave: Found Because the Rubble Stopped Coming Back

26 September 2026 Last checked 2026-09-26

Royal Engineers cutting an escape route for a hospital that never took a patient broke into a cave nobody knew was there. It ends 76 metres down at a lake of clear water.

In 1942 a Royal Engineers tunnelling party inside the Rock realised they had hit something because their rubble stopped coming back. The spoil from the blast was falling away into a void on the far side of the rock face. What they had opened, by accident, was a chamber system that runs 76 metres below the entrance and ends at a lake of clear water 37 metres long.

That cave is still down there, and you can still get into it, but not in the way you get into the famous one. This is the part of St Michael's that almost nobody sees, and the gap between the two halves of the same cave system is the most striking thing about it.

What they were digging for

The upper cave had been fitted out as an emergency military hospital. The plan in 1942 was to cut a second entrance into its lower chambers, partly for air circulation and partly as a way out if the main entrance were hit in an air raid. That is the job the engineers were on when the rubble vanished.

Two facts sit oddly together here. The hospital was fully prepared, and according to Wikipedia's account of St Michael's Cave it was never used as such. No patient was ever treated in it. The escape route being dug for a hospital that would never open is what opened the cave.

The Royal Engineers were not casual visitors to the inside of this hill. The same period produced the WWII tunnels, the Great Siege Tunnels extensions and the sealed observation chamber of Operation Tracer, where six men volunteered to be bricked in. Gibraltar's wartime tunnelling is measured in tens of kilometres. Finding a cave while doing it was luck, not exploration.

What is actually down there

The numbers are worth stating plainly, because most descriptions of this place are adjectives. According to the entry for New St Michael's Cave, the descending chambers reach a depth of 250 feet, or 76.2 metres, below the level of the entrance. The lake at the bottom is nearly 40 yards long, which is about 37 metres, and holds an estimated 45,000 imperial gallons, roughly 200,000 litres.

Two hundred thousand litres is a swimming pool's worth of water sitting inside a limestone hill 300 metres above the sea. Put the volume and the length together and what you get is something broad and shallow rather than a pool you could dive into, which matches every description of it. Its interest is that it is undisturbed and glass clear, which is what happens to water that has been filtering through limestone with nobody stirring it.

The chambers in between carry what speleologists describe as examples of almost every known cave formation. That claim is repeated everywhere and we could not trace it to a published survey, so treat it as the guidebook line it is rather than a measurement.

Why the Rock is full of holes

None of this is a freak. The Rock is limestone, and rainwater has been dissolving its way through that limestone for a very long time, which is why the Rock of Gibraltar holds more than 150 known caves in a territory of 6.8 square kilometres. St Michael's is simply the most visited of them. Gorham's Cave at sea level on the eastern side is the most important archaeologically, and dozens more are numbers in a survey that no member of the public will ever stand inside.

That density is also why a wartime tunnelling programme kept running into natural voids. Cut enough kilometres of gallery through a hill like this one and you will eventually open something. In 1942 the something was worth keeping.

The 20,000 year claim

You will read that the lower cave had been sealed for around 20,000 years before 1942. It is a striking figure and it may well be right, but we could not find the primary source it comes from, and we are not going to repeat it as fact.

What is defensible is the structural point. The chamber had no known entrance until the blast made one, and nothing in it had been disturbed by people. That is the reason the tour is run in small groups with a guide rather than opened as a walkway, and it is why there is no lighting scheme, no handrail and no gift shop at the bottom.

The upper cave gets close to a million visitors a year. The lower cave takes ten at a time.

Upper and lower, side by side

Getting a place on it

The trip runs by arrangement rather than on a timetable. Groups are capped at ten and will not usually run below five, children under ten are not taken, and three days of notice is the standard requirement. The contact is published on the official visitgibraltar listing for the cave, and it is a named local caver rather than a booking desk, so treat it as a phone call you make in advance and not a ticket you buy on the day.

Price is the part nobody can pin down. Third party listings disagree with each other, with figures from a few pounds through 25 GBP and up to 80 GBP for packages that bundle other attractions, and the trip is not priced at all on the Nature Reserve's own ticket page. We wrote the same caution into our guide to the Rock's bat caves: confirm the figure when you book, rather than turning up with a number you read online.

For reference, general reserve entry is 30 GBP for an adult and 22 GBP for a child aged 5 to 11, under fives free, with the reserve open 09:30 to 19:00 from May to September and 09:00 to 18:00 from October to April. That ticket gets you the upper cave. It does not get you the lower one.

Who should not book this

We would rather say this than have somebody waste a morning. If you do not like enclosed spaces, this is the wrong trip, and it is the wrong trip in a way that a torch and good intentions will not fix. The route narrows, you go down on rope in places, and turning back partway is awkward for the whole group.

Bring footwear with grip that you do not mind wrecking, and clothes the same. It is muddy. The temperature underground stays moderate and stable year round while the surface does whatever Gibraltar is doing that week, so the Levanter that ruins a viewpoint morning makes no difference at all to what is happening 76 metres down.

The legends, and what is left of them

Two stories attach to this cave and both are worth knowing, mostly because both are wrong.

The first is the tunnel to Africa. The story holds that a passage more than 24 kilometres long runs from St Michael's Cave under the Strait to Morocco, and that the Barbary macaques walked through it to get here. In one version the macaques go back down it to die, which is offered as the explanation for why dead ones are rarely found. Showcaves puts the geological objection simply: a passage like that is extremely unlikely to exist, and if it did it would be full of seawater. The real story of how the macaques got here is a better one, and we have told it in our post on the apes legend and the 1942 order to restock them.

The second is the Bottomless Pit, the shaft in the upper cave that gave Gibraltar one of its favourite legends. It has a bottom. Showcaves describes it as a rather unimpressive shaft of about 8 metres. Being able to say that is fairly recent in the cave's long history of being visited, and it is a reminder that almost everything spooky about a cave turns out to be a measurement somebody had not taken yet.

What the upper cave is now

Do not skip it because the lower cave is the more interesting story. The main chamber, the Cathedral Cave, has been an auditorium for decades, and since 2021 it has carried an immersive light and audio installation called The Awakening, built by the Gibraltar firm Wright Tech Media with the light specialists Pixel Artworks and announced by the Government of Gibraltar in 2021. The reserve's own St Michael's Cave page is honest about the access problem: there is a viewing point for the show near the exit, but the cave cannot be made step free without damaging what people come to see.

One number we cannot settle: the seating capacity of the Cathedral Cave auditorium. Wikipedia says over 100. Several tourism pages say around 400. We found nothing authoritative enough to choose between them, so if you are planning an event there, ask the reserve directly.

Getting to the cave at all in 2026

The cable car has been closed since 18 November 2025 for a full rebuild and is not expected back until 2027, which our cable car status guide tracks. St Michael's Cave sits over 300 metres above sea level in the Upper Rock Nature Reserve, so the options are a licensed taxi tour, the reserve shuttle sold as an extra, or walking up. If you are walking, the Mediterranean Steps is the hard way and the direct paths from Jews' Gate or Devil's Gap are the sensible ones.

Allow more time than you think for a lower cave booking day. Three hours underground plus the climb or the taxi either side is most of a day, and you will not want to schedule anything demanding after it.

If you are making a single trip up the Rock, pair the cave with the southern viewpoints rather than the town. The summit and Europa Point are where the sightlines across the Strait open up, and doing the underground and the long view on the same ticket is the most efficient day the Upper Rock offers.

Victory Suites runs 100+ apartments in Gibraltar on Devils Tower Road, about ten minutes from the frontier, with rates in GBP and listings across 300+ booking channels. If you are planning a lower cave morning, book the cave before you book anything else that day, because the three day notice rule is the part that catches people out.