A Mysterious Network of Tunnels Were Once Dug Beneath Europe - And Scientists May Finally Know Why: Some suggest these passageways once had a spiritual or religious function, while others claim they’re relics of long-departed aliens.

As a boy, Raimund Edlinger loved exploring caves. Lurking beneath the surface, dark and unknown, they sparked wonder and awe. Now a robotics expert at the University of Applied Sciences Upper Austria, Edlinger works on search-and-rescue robotics. So when his colleague, Kurt Niel, asked him to join a nearby erdstall tunnel mapping expedition last year, Edlinger readily agreed. Erdstall tunnels are mysterious underground passageways numbering in the thousands and scattered across Europe—but they’re anything but ordinary caves. Consisting of claustrophobically narrow tunnels punctuated by squat chambers, erdstall complexes can be unexpectedly intricate. Some span multiple levels, connected via schlupfs (German for “slip”), or tight crawl spaces that are just over one foot wide. To move from one level to another, an intrepid explorer must squeeze through openings barely large enough for a single person.
To illustrate the extreme environment of erdstall tunnels, Niel, a professor of machine vision at the University of Applied Sciences Upper Austria, shared a video on YouTube in which he pants with effort as he lowers himself down a schlupf, his torso brushing up against its smooth sides, his overalls dusted with dirt.