This tunnel in Croatia is much larger than the one Edlinger has explored (Image: Getty Images)
This terrifying network of underground is enough to make you fear for your life – with its small and enclosed spaces, no one is quite sure why they were built.
The has been explored by brave cavers wanting to see the structure. Some believe the network has a religious function while others recognise the mysterious nature of the caves.
Raimund Edlinger has explored the caves and filmed what it is like inside, and the footage is enough to make you feel claustrophobic even just by watching through a screen.
He is a robotics expert at the University of Applied Sciences Upper Austria and he mainly works on search-and-rescue robotics. With the university, he has been tasked with the these mysterious caves.
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Mapping these is no easy job, it is a difficult and dangerous task – the tight crawl spaces and small passages mean that the tunnels are often barely big enough for a person to fit through.
Knowledge about the tunnels is sparse and while there are many similar variations of these throughout Europe, not much is known about them.
The erdstall shed light on an ancient past and an age of human civilization that we now know very little about. Some historians have argued that the tunnels were used as granaries or hiding spots from looters.
In some tunnels, fireplace-like structures and vent pipes which reach the surface have been found, suggesting these places could have been used in the winter when the surface was very cold.
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is not easy work but technology has helped researchers to gain an understanding of these underground spaces. Recently, researchers have begun to use 3D imaging to draw up a digital plan of the caves.
They were able to find a carved-out space that could accommodate a wooden door and two holes in the opposite wall. They also discovered posts seated in the holes which could have been used to barricade the room from intruders.
Edlinger hopes to be part of research into other caves across Europe. Archaeologists have completed research to understand how long it would have taken people in the Middle Ages to build these . They estimate that it would have taken three to five people several months to complete one of these structures.
The research into the mysterious structures remains ongoing.