Southwest China is riddled with caves. The sheer number of them means most remain unexplored, despite caving becoming increasingly popular in China. The cave system is home to all sorts of wildlife, and now one more previously unknown species has been added to the list.

Dr Peter Mortimer, a South African mycologist and professor at Kunming Institute of Botany, is an accomplished caver and climber and he took us down one of his projects about 20km north of Kunming – Yanzi Dong, or Swallow Cave.

Rappelling using a climbing rope took us into the main chamber of the cave. Ropes are the preferable mode of transport to scrambling down a rotten tree trunk dropped into the shaft, as locals used to do to harvest stalagmites in the cave. Delicate Xylaria fungus now covers the trunk entirely, its fluorescent glow in the headlamp made the descent feel like a dive into oceanic depths.

Spooked bats flapped around and the air was thick with the smell of their excrement. Down on the floor, masses of ghostly white millipedes feasted on thick piles of bat droppings. On the walls, cave leeches tried to creep up on the sleeping bats. In the light of headlamps the leeches looked blue.

The descent into the cave by rope is not for the faint-hearted. Photo: Pavel Toropov

Mortimer found a fluffy white ball with bones sticking out of it – a dead bat being devoured by a fungus. He packed the carcass into a plastic bag.

“I will get its DNA sequenced – this fungus may well be new to science,” he said. Yunnan Province has the highest biodiversity of fungi in the world and Mortimer’s research group has identified over a thousand new species.

Fungus eating a bat – it turns out to be a new species of fungi. Photo: Peter Mortimer

Caves can harbour very sinister inhabitants – fungus Histoplasma capsulatum, which lives on bat droppings, can cause the potentially fatal “caver’s disease” – histoplasmosis. Also, the virus which caused the 2002 SARS (Severe Acute Respiratory Syndrome) outbreak is believed by some experts to have originated from bats in a cave in Yunnan.

Stumps of cut stalagmites indicate how far the locals have gone, but without specialised equipment one can go only as far down as the main chamber. The lower levels of the cave are accessible only by a series of rappels through a narrow shaft recently discovered by Mortimer.

“This part of the cave is totally safe,” said Mortimer after we had explored the thankfully wide corridors and spacious, stalactite and stalagmite-filled main chambers. “Let’s go deep down to the underground river, it is a much more full-on experience.”

Millipedes feed on bat droppings. Photo: Pavel Toropov

It is very easy to run into serious trouble in these caves. The rappels took us through piles of giant blocks held together by their own weight or glued into the walls with mud.

Everything was caked with mud.

“This is like a reverse birth” laughed Mortimer, as he squeezed down through a particularly narrow and muddy tunnel.

We rappelled and squeezed down into another chamber – a stream flowed into it before vanishing into a wall. The low ceiling was studded with large, loose blocks. We were now about 100 metres below the surface.

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“We have once tried to follow this stream up, we could see the light – there is an opening – but it was blocked by rockfall.” Mortimer said. “Let’s try to follow the stream down for a bit.”

We did not get far – very soon the ceiling and the floor converged to such an extent that Mortimer decided to turn around. It is rainy season and a rainstorm can flood this passage in minutes.

“We should come back later,” Mortimer said. “It would be great to find an exit from here, this would save us going up the ropes to get out.”

Locals have harvested stalagmites in the main chamber, leaving stumps and scars. Photo: Pavel Toropov

Ascending a vertical rope requires considerable physical effort as well as technique. In the muddy, narrow darkness, where a thrashing, tensioned rope or a wrong placement of one’s foot may trigger a rockfall, it was an uncomfortable, and seemingly interminable, experience.

Driving back to Kunming, Mortimer kept pointing out limestone formations in the surrounding mountains: “There are caves there also, all worth exploring.”

Chinese cavers have been exploring caves in Yunnan and neighbouring Guizhou for a while and there is a caving club in Kunming.

Incredible stalagmites abound in the extensive cave systems of Yunnan. Photo: Peter Mortimer

“Those guys are legit,” Mortimer said. “They can take you on a real caving adventure. There is an underground river in Guizhou that you can run in a raft, for two full days, all inside a cave system.”

Our brief foray into Swallow Cave was quite an adventure already – less than two days later, when we were safely back on the surface, a minor earthquake rattled Kunming. We wondered how many loose blocks it dislodged down below.

The bat-eating fungus we found turned out to be a very unusual beast indeed. The DNA sequencing results showed it was a completely new species, related to the famous Cordyceps sinensis, which infects caterpillars and is highly prized in Chinese traditional medicine, but this was the first time the fungi of this family was found to be feeding on mammals.

This find surprised and excited even Dr Mortimer.

“Molecular evidence shows it to be very isolated from its nearest relatives, and it is also morphologically totally distinct from anything I have seen before,” he wrote.

Yunnan’s caves, so numerous and accessible, are full of surprises and discoveries.