Tunnel.

Churchyards and private farmlands throughout the German state of Bavaria are perforated from below by "more than 700 curious tunnel networks" whose "purpose remains a mystery."

As Der Spiegel reports, "The tunnel entrances are sometimes located in the kitchens of old farmhouses, near churches and cemeteries or in the middle of a forest. The atmosphere inside is dark and oppressive, much as it would be inside an animal den."

Although the subterranean networks are considered an "extremely unusual ancient phenomenon," other "small underground labyrinths have been found across Europe, from Hungary to Spain, but no one knows why they were built."

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