The Tivoli Cave in the city of St. Gallen is a small cave in the middle of a beautiful forest. The best way to get there is to take the bus to the "Hagenbuch" stop. Or take the wonderful panoramic path and then descend the Tivoli stairs. Please respect the privacy of the private property there! Continue directly to the right along the small path and DO NOT go left/straight across the small bridge.
If you go all the way in (bring a headlamp), you won't get very far. It's almost a bit spooky there. If you look up, you'll see a shaft about 13 meters long that leads straight to the very top. This is where buckets used to be lowered to fetch water. The hole at the top was eventually sealed and covered with earth and no longer exists. Therefore, there was no escape route for the nuns. There used to be a rickety and very rotten wooden ladder here. This has since been removed for safety reasons. It is a man-made cave; clear marks made by tools have been discovered in the rock.
The cave is about 17 meters long and between 6 and 7 meters wide.
Source: thisismysaintgallen.com/tivolihoehle/