The atmosphere inside is dark and oppressive, much as it would be inside an animal den.
Suddenly the farmer could no longer hear anything from above.
The tiniest passageways, known as "Schlupfe" ("slips"), are barely 40 centimeters (16 inches) in diameter.
The ground beneath the southern German state of Bavaria is literally perforated with these underground mazes -- and no one knows why.
Three members of a group called the "Working Group for Erdstall Research," wearing red protective suits and helmets, dragged the heavy concrete plate away from the entrance and disappeared into the depths.
Their leader, Dieter Ahlborn, began by crawling through a passageway only about 70 centimeters (2 foot 3 inches) high.