Marek Audy has many world discoveries behind him: from salt caves in Iran to extensive sandstone cave complexes in Venezuela. Last year, he became the first person to stand in a newly discovered Albanian system of hypogenic caves, where together with other expedition members they mapped several thermal lakes, including the vast Neuron Lake, named in honor of the foundation that supported the research.

You’ve been through dozens of caves. What still took your breath away during the last expedition?

A big surprise was the Sulfur Cave. The air temperature there stays around 30 °C and humidity reaches almost 100%. It is inhabited by an incredible number of insects and spiders. We discovered a web with an area of 100 square meters and about 10 centimeters thick. We counted around 60,000 house spiders in it. I would definitely not recommend entry to people with arachnophobia. The first time I entered the cave, I was almost naked with only a headlamp. Walking in flip-flops among spiders, scorpions, and centipedes wasn’t exactly pleasant.

So your discovery also attracted the attention of biologists?

Yes, in the following years we were joined by biologists from Romania, the USA, and Italy. They found that a unique biotope without natural predators had formed in the cave. For example, spiders there lay only a third of the eggs compared to normal environments. A colleague aptly commented: “Here they have an eternal party.” The females just sit, don’t stress, and wait for whatever flies into their mouths.

How did it happen that this cave complex remained undiscovered for so long?

The reason was strict restrictions from the times of Albanian totalitarianism. Entry to the border area was forbidden from both the Greek and Albanian sides. Thermal mineral water was piped from the springs to the spas over several kilometers. The area long remained outside any scientific or public interest.

When did the idea arise that something extraordinary might be hidden there?

During a rafting expedition in Albania, I thought at the Langarica thermal springs that there could be hypogenic caves here. These do not form with water flowing down from the surface but are instead created by mineral waters pushed up from great depths. The mineral water is then saturated with hydrogen sulfide, which is released on the surface, mixing with oxygenated water from above. This creates sulfuric acid, which aggressively corrodes limestone and transforms it into gypsum. Domed chambers are characteristic of this type of underground. Precipitated gypsum creates very beautiful speleothems here: crystals, gypsum roses, etc. On the walls above the lakes there is no shortage of pure sulfur. Hypogenic caves have a special, attractive atmosphere.

We discovered the mentioned Sulfur Cave that same summer. Right beyond the entrance, a lake blocked further progress. In the autumn, together with cavers, we mapped the Sulfur Cave. At Christmas, while sorting drone photos from the area, I noticed clouds of vapor rising from the forest above Sulfur. In February, we returned to the site and found a deep abyss. But 100 meters of rope was not enough to reach the bottom. We only saw something like a pond below us, which we managed to reach only in 2023.

You must have been impatient…

It takes time to synchronize schedules, the team, and permits. Moreover, we misjudged the depth – when I first rappelled down, I thought I was five meters from the surface. It turned out to be thirty, which of course I couldn’t jump. So we had to pack up and return with longer ropes. Anchoring in walls covered with gypsum is also not easy. Gypsum is soft, and you need to find solid limestone massifs.

What was it like to stand in the Atmos Cave for the first time, where you discovered the unique lake?

The cave is gigantic and very unusual. The first thing that hits you is the strong smell of hydrogen sulfide – its concentration can be hazardous to health. Even if it is only a few ppm, you know that if something goes wrong, you might not survive. At first, we carried two-liter diving tanks with air, but later we stopped. We knew that in a real emergency, they wouldn’t help us anyway. Today we have a strict rule: when the detector goes off, we act immediately.

How did you feel when you first saw Neuron Lake?

It was euphoria. We saw a vast water surface and couldn’t see the end. We had a laser rangefinder with a reach of 100 meters – it showed nothing, just an error. I thought it was because of the mist. But it worked on closer points and measured 95 meters. No further.

And you decided to measure it yourself…

I stripped and went into the water. Because of the hydrogen sulfide, my colleague stuck a detector onto my helmet. After 30 meters, I still couldn’t see the shore. As I stirred the water, hydrogen sulfide started to release. The detector triggered a warning, then an evacuation alarm. I got nervous. I had spent a few years as a designer in chemical plants, and thanks to my knowledge from FME BUT, I knew that by stirring the water the situation could quickly worsen and my colleague waiting on the shore must not come to help me.

What happened next?

I turned around and slowly swam back. My colleague reassured me to swim calmly. I didn’t feel anything, but the risk was real – in such an environment, pulmonary edema can occur even several hours after exposure. Luckily, nothing happened to anyone. But for a week after returning, I still smelled like hell. We agreed that we’d better not repeat such ideas.

How did you finally manage to measure the lake accurately?

We returned with a boat and a mobile lidar scanner. After several measurements, we scanned the entire cave and created a very dense point cloud. From that, we generated a 3D model. The resulting accuracy is within one centimeter at every point. Brno-based Geonika helped with the data processing. The Czech Hydrometeorological Institute provided a sonar, with which we mapped the lake bottom profile. Hydrogeologists from Charles University carried out tracer tests and determined where the water from Neuron Lake flows out.

It’s the largest known thermal lake. Which is the second largest?

Neuron Thermal Lake, with an area of 3200 m², is truly gigantic. Its primacy is beyond doubt. The second place is debatable. However, it is likely that the second-largest cave thermal lake in the world is the much smaller lake in Turtle Cave, with an area of 360 m². It is in the same area, and our hydrogeologists found that it is the same water flowing through unknown channels from Neuron Lake.

How do you sleep when you discover something big but can’t verify it immediately?

One is excited but knows he can’t do it alone. You must convince colleagues – geologists, biologists, mineralogists. No discovery is made without a team. Neuron Lake remained hidden for a long time, even though its water flows into Sulfur Cave – and that is visible and easily accessible.

Before the expedition, I came across an old study by a Ukrainian hydrogeologist. He claimed that the mineral springs flow out of Sulfur – and for him, the story ended there. He had been on the site but never went further. Maybe he didn’t have a headlamp. Even renowned experts sometimes overlook fundamental things.

How did your technical education at BUT help you?

I fled from chemical plant design, today I work in graphics and books. But engineering thinking stayed with me. It helps me with planning, dealing with authorities, and publishing. For example, thanks to an article in the February issue of National Geographic, the whole world learned about our discovery. And that also sparked the interest of Albanian politicians.

Where is your next expedition headed?

We are not leaving the Balkans. It is an extremely interesting area, full of unexplored caves. In the Langarica region, there are large hydrothermal systems that no one has entered yet. We cooperate with local authorities and scientists, and I believe many fascinating discoveries still await us.

(mar)