If a place has received 31 million visitors over the past nearly 200 years, it clearly is something special. Equally, if the thought of becoming submerged in what could be regarded as mass tourism horrifies you, will it really be worth it?
After visiting the Postojna Caves in southern Slovenia, I am not much closer to answering that conundrum.
These are the most visited caves in Europe and one the largest classic "karst" cave systems anywhere. There are 20 kilometres of natural passages, galleries created by the Pivika River about two million years ago. The caves were first explored properly in the early 1800s and the first tours were operated not long after. Back then the only way for visitors to reach some of the most spectacular caverns and arrays of stalactites and stalagmites was on foot by torch light.
Today a small electric train transports visitors deep into the caves which are now lit by electricity. Even an early arrival at the cave entrance won't save you from long lines of visitors waiting for the trains. This place has a long and busy season.
The train "carriages" are basic metal seats with no sides and I expected that progress through the tunnels to the starting point for the walking tour would be rather sedate. I was wrong. The train hurtles through the often quite narrow and low passages - this is one place where warnings to keep one's limbs inside the train and for tall people to be prepared to duck actually mean what they say.
My heart sank when I saw how many people were milling around in the cavernous semi-dark muster point at the end of the train journey. The cave echoed with guides trying to track down their particular language groups and the shouts of cave officials trying vainly to stop people taking flash photographs.
But amazingly, about 15 minutes after the various groups started to wend their way up the steps of Big Mountain, everyone had become so scattered along the trails that at one point I found myself entirely on my own. Around me were carefully lit towers of stalactites and stalagmites, more spectacular and massive than any others I've seen around the world. There was no sound at all, not even the drip of water... and then the lights went out.
Yes, there were hundreds of people down here with me, somewhere. But it was still more than a little eerie. I tried to stop myself trying to recall just how many metres I was underground and thus how many tonnes of mountain were above me. The lights came back on just as I was comforting myself with the thought that the various speleothems, some delicate as strands of spaghetti, had been undamaged by even the merest of rockfalls over millions of years.
Apparently, so the cave guides tell you, the lights are turned off deliberately during each visit just to give you a sense of how dark the caves are. Not sure if that's a case of "Yeah right" or not.
Once you lose the crowds and stop worrying about succumbing to mass tourism the caves are genuinely magical. The colours of the limestone vary from brilliant white to striped, marbled forms in pale golds and pinks. While some formations look like crocodiles, vegetables and castles others are just simply astonishingly beautiful works of nature.
At one point I emerged from a tunnel high above a natural cavern and looking down saw half a dozen men wearing long black cloaks, standing quietly behind a rock. They looked decidedly sinister. It turned out they were an acapella choir waiting to perform in a nearby cavern. By the time I descended the steps towards them they were singing Slovenia folk songs, demonstrating the beautiful acoustics of the cave.
While humans have only ever made fleeting visits to the cave, the Postojna cave does have some permanent residents, mostly notably the very weird amphibian known as the "human fish".
An aquarium has been installed in one of the caves so that visitors can see several specimens of Proteus anguinus. This is a variety of salamander, is totally blind, has a long tail fin and four legs, has gills but also simple lungs for breathing out of water and has rather creepily human-like unpigmented skin.
No-one has ever seen its means of reproduction. The Proteus anguinus in the aquarium lay slug-like along rocks, glowing ghostly white under faint lighting, apparently unconcerned that their sex lives remained a mystery.
The walking tour ends in the Conference Hall which is so large it can accommodate up to 10,000 people for musical performances. The trains meet visitors here and propel them back out into the light and the maelstrom of milling crowds, shops and restaurants. It's a bit of a shock to the system.
Would I go in again? Probably... especially now that I know that it's possible to somehow lose the shuffling crowds and bellowing officials. But I still can't reconcile the uneasy feeling that sadly all around the world the sheer volume of people wanting to see such stunning sights become at the same time the weapons of, if not their destruction, then their potential degradation.