Lost caves are what the name implies: the knowledge about a cave still exists, but the cave itself has gone missing. In other words, these caves or cellars are very well known, sometimes even famous, they definitely existed. In most cases they were even once open to the public. But today you can no longer visit them, but not because they are simply locked. They are not accessible because they have been lost, nobody knows where they are.
It is likely that they are not completely lost. There is probably someone who still knows where they are, but it is not published, not common knowledge. If you ask at the tourist office, they will tell you that you are wrong, and it does not exist. And they are quite astonishment when you pull out the 19th century Baedecker guidebook, and they read the description. In some cases, the cave is simply not accessible because it is on private property and the owner does not reveal anything. But there are more and more caves that have actually been destroyed because something was built on them, because the whole mountain was quarried, because there was the danger of collapse, or because something was disposed of in the cave.
showcaves.com is dedicated to underground tourist attractions. Strictly speaking, these lost objects are not among them, because they are not open for tourists. But we also deliberately list closed show caves, of which there are very many, precisely because their non-existence makes them impossible to find. No one pays for webspace, no one advertises that their cave is bankrupt. And so for years there are still brochures and travel guides and great photo galleries of visitors, but when you get there it's closed. Lost caves are even worse: you can't find them because they've even forgotten where they are, or because the place where they should be no longer exists.
Caves are threatened in their existence by two diseases of civilisation. Raw material extraction: both limestone and gypsum are raw materials and these are extracted in quarries. If a cave is discovered in the process, it is simply mined along with it. In other words, the Neanderthal Cave was destroyed for the important reason of making road gravel from it. Buildings: if a building is erected, it needs a stable foundation. If a cave is found, it must be stabilised in such a way that the foundation is not affected, which usually corresponds to filling the cave with concrete. This applies to any type of building and of course includes roads and railway lines.