- Visitors to the forested Gunung Sewu geopark in Yogyakarta province are lowered 60 metres to the cave floor, where plants from ancient times still grow
- Strict limits on tourism keep the caves pristine. Come at the right time and sunlight streams through a hole in the roof of on cave to make an Instragram moment
The air is quiet except for the sound of my breathing as I hold nervously onto a single rope, dangling at a height of 60 metres (200ft), before being lowered slowly into the dark cave by an instructor. It's my first time vertical caving, but I am safely clipped into a harness, and my tension soon turns to awe as I observe the towering, plant-covered cave walls.
Before long, my feet touch firm ground, and I am released from my harness by guide Suharjono. Our group of 20 tourists murmurs with excitement as we land in this still natural cave in Indonesia's Yogyakarta province, equipped with boots, helmets and headlamps to help us negotiate the dark, slippery terrain.
Jomblang cave has become a popular adventure destination for Indonesians and foreign visitors to the region in Java. The main space of the cave is about half a football field wide, and overgrown with various types of mosses, ferns, shrubs and trees.
Suharjono says the cave is formed from a doline, or sinkhole, that appeared thousands of years ago in the forest floor and brought a layer of soil along with vegetation crashing down. The sinkhole has a 50-metre wide lip. The vegetation continued to flourish.
"They are called ancient forests, because the species of plants here are no longer found elsewhere outside the cave," says Suharjono, who uses only one name. "For example there are plants like chillies here, that when eaten taste sweet," he says, adding that the plant has not yet been named.
Jomblang cave is one of thousands of such caves in the karst hills of Gunung Sewu (Thousand Mountains), in a forested region on the coast of Gunungkidul, a district in Yogyakarta province.
Inside the cave it is dark and humid. The guide invites us to make our way along a 300-metre passageway lit only by our head torches, to another cave, Grubug.
The soil is muddy and slippery because of the humidity and recent rain, so we hold onto a rope that runs along a wall of the passageway, stopping every now and then to admire beautiful stalagmites and stalactites.
When we arrive at the Grubug cave, we hear the reverberating roar of an underground river. A bright, vertical beam of sunlight cuts through the darkness. Called the "light of heaven", the beam enters through a large hole in the ceiling of the cave and penetrates all the way to its bottom.
The light only appears for one to two hours during the day, from 11am to 1pm, and draws visitors who come to take photos of the silhouettes it creates.
The manager of Jomblang cave, Budianto, who uses only one name, says aside from Instagramers and adventure seekers, the cave also draws Indonesian researchers interested in its biodiversity.
"We want to preserve this cave and its ancient forests, so not many tourists are allowed into the cave each day," says Budianto. Visitors are limited to a maximum of 80 people per day, and if it is low season, as few as 20 people a day might visit.
Before exploring the cave, tourists can wait in a pavilion surrounded by gardens filled with flowers and coconut trees. Afterwards, the cave managers provide them with rice and side dishes of tempeh, chicken and sambal.
Since the Gunung Sewu area was given Unesco Global Geopark status in 2015, Jomblang cave has become the geopark's highlight. But the geopark offers other adventure tours too, such as "cave tubing", in which guests mount a tyre to ride down subterranean rivers in the Pindul and Kalisuci caves.
Because of the strict regulations, the caves remain pristine and peaceful, says Budianto. "Cave tourism has become a source of community income, therefore we must continue to ensure the caves are not damaged by the tourists who come to see them."
How to visit the caves: If you and your party have their own equipment, you can enter Jomblang cave without paying. However, if you need to rent equipment and hire guides, the cost of a visit ranges from 450,000 rupiah to 1 million rupiah (US$33-73) per person depending on your requirements.
Getting there: Jomblang cave is about 60km (1.5 hours) from downtown Yogyakarta and can be reached by motorcycle or car. The cave (goa-jomblang.com) opens daily from 8am to 2pm, and the best time to see it is early afternoon. Several tour operators provide package tours to the cave, including Maioloo Tour (maioloo.com).
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