Most trips to this region are organised around monuments and the city. This one is about something else: getting inside the geology, in a place where the ground collapsed and what was on top carried on growing below.
01 A hole with a forest inside
The formation is a collapse doline: a cavity hollowed out by dissolution of the limestone until the roof could no longer hold and came down, leaving a shaft open to the sky with vertical walls and a cone of debris at the bottom.
The extraordinary part is what happened afterwards. At that bottom, some sixty metres down, with constant humidity and overhead light, a forest established itself: ferns, moss and trees growing upwards towards the mouth of the hole.
And the descent is not a staircase or a lift. It is done on a harness and rope, using single-rope technique, with the system rigged and controlled by the site's operators. No previous experience is needed, but you do need to be willing to hang on a rope.
Sixty metres of drop and, at the bottom, a whole forest that has never seen the horizon.
02 The shaft of light at mid-morning
Once down, you walk along a passage to a second chamber with its own opening in the roof. That is where what almost everyone comes to see happens: a solid shaft of light that drops from the hole and lands on the cavern floor.
The effect depends on two things you do not control: the sun being high and the sky being clear. In practice, the good window runs from ten to twelve, and that is why the whole day's schedule is built around those two hours.
It helps to keep expectations calibrated. The shaft is not a constant spotlight: it appears, moves, weakens when a cloud passes and vanishes entirely if the sky is closed. You can complete the whole descent and not see it.
03 Eighty places and a single departure
Logistics are what ruins most trips here, so they are worth being clear about. The daily quota is around eighty people, there is a single group departure in the morning, around half past nine, and entry stops admitting people at about ten.
That means arriving at eleven is not being late for a queue: it is not descending at all. And it means that, in high season, the quota can fill before that hour, so booking ahead stops being a convenience and becomes the only way to be sure.
Since the site is a couple of hours from the city on mountain road, the arithmetic is obvious: you have to leave before dawn. Local operators organise the transfer for exactly that reason, and it is not a dispensable luxury.
04 Mud is part of the deal
Nobody warns about this enough. The bottom of the shaft is a cone of debris with vegetation and permanent humidity, and the path to the second chamber is real mud, the kind that gets into your boot. Rubber boots are provided for a reason.
In the dry season, May to September, the mud is manageable and the light appears often. Between November and March, with monthly rain between 237 and 392 millimetres, the descent stays open but the ground is far worse and the shaft of light usually comes to nothing.
And there is a hard limit: if a heavy storm falls, the descent is suspended. That is not a commercial decision and it is not negotiable; it is the only sensible one in a karst cavity with an underground river beside it.
05 What is around it
The shaft is not an isolated oddity. It sits inside a karst region of some 1,300 square kilometres, with on the order of 40,000 conical hills, recognised as a global geopark since 2015 and long known as the thousand mountains.
That landscape works the opposite way to the rest of the region: there are no surface rivers because all the water travels inside, through a system of passages, shafts and underground channels. The district has historically been water-poor for exactly that reason.
That is why it is worth giving the area a whole day and not just the morning. The zone has more visitable caves, cliff beaches on the southern coast and a relief of rounded hills that looks nothing like the rice fields you left behind.
06 How to put it together
The plan that works is to book ahead, leave the city around five in the morning and count on being back by mid-afternoon. The activity itself takes a few hours, but the transfer and waiting for your turn eat the rest.
Take clothes you can write off, spare socks and a protected camera. And do not fixate on the shaft of light: the vertical descent and the forest at the bottom justify the day even if the sun does not cooperate that day.
And a final warning about the calendar: quota, timetable and price conditions change frequently, and depend on the operator. It is worth confirming everything the week before rather than trusting what you read months ago.
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