"According to Standart daily newspaper under the Bulgarian capital Sofia there's a thousands of kilometres of tunnels built between Roman times and Communist regime. They shelter about 20 000 people - most of them are criminals and bums."
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects