By ErgoSum88 via Wikimedia Commons

About a thousand people worked in the SubTropolis, an industrial park located in an enormous excavated mine which is as bag is 140 football fields. It opened in 1964 and lured tenants low energy costs and cheap rents, according to Bloomberg. The walls are carved out of 270-million-year-old limestone deposits and help keep humidity low and temperatures at a constant 68 degrees. Rents run about $2.25 per square foot.