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. 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](https://en.wikipedia.org/wiki/St._Michael%27s_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](/blog/operation-tracer-stay-behind-cave-gibraltar), 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. The numbers are worth stating plainly, because most descriptions of this place are adjectives. According to the entry for [New St Michael's Cave](https://en.wikipedia.org/wiki/New_St._Michael%27s_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. 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](https://en.wikipedia.org/wiki/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. 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 St Michael's Cave | Lower St Michael's Cave | | How you get in | Walk in on the Nature Reserve ticket | Booked trip with a guide, three days notice | | Visitors | Close to a million a year | Groups of five to ten | | Time inside | As long as you like | About three hours | | What it involves | Steps and a paved route | Scrambling and some rope work | | Minimum age | None | Ten | | The set piece | The Awakening light and sound show | The lake, 76 metres down | 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 list