Deep beneath the modern cities of Europe lie thousands of mysterious tunnels. Known as erdstalls, no one is certain who built them, but theories surrounding these strange underground labyrinths invoke everything from secret societies to cryptid creatures.

Mike and Martin explore the origins of these darkened places, and shine a light on one of the world's most puzzling historical mysteries.

Website: talkingtilldawn.com

Twitter: @TalkingTill

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Further reading/sources:

https://docs.google.com/document/d/1wpkST2xMiFDrr9-bAuI4uXqiqALZHcVkPPKgfhyJcY4/edit?usp=sharing