The Initiation Well in Sintra, Portugal
Quinta da Regaleira, the estate where this well or “inverted tower” is 27 meters deep, and just like the rest of the estate it is packed with symbolic references to cosmology, alchemy, and mythology (mostly roman and greek), and perhaps even mentions to freemasonry. The gardens are huge and constantly well-kept, and are the best and biggest part of the 2-3 hour visit.
The owner of the estate as we now know it was 1800’s Carvalho Monteiro, who on the last floor of his palace actually had a secret door that led to his alchemy lab. He commissioned the Italian architect Luigi Manini to design pretty much all there is in his estate, and Manini did a splendid job, everything being thought out in an astonishingly detailed manner. There are waterfalls from which you can walk behind the falling water, lots of grottos and manmade tunnels to explore, majestic towers,and beautiful fountains.
One can reach the bottom of the Initiation Well after walking for a good 5-10 minutes below ground in near absolute darkness, although they recently installed some ambient lights down there. (Source)