The city that never sees the light...

[keithisco]

Madrid, like Rome, was built on 7 hills; Madrid, like Rome, has hundreds of kilometres of long forgotten tunnels (catacombs). Some of these excavations are known about, but the vast majority remain to be discovered and explored - they date back well over 1000 years and who knows what discoveries are to be made...

READ MORE (Courtesy El Pais): http://elpais.com/elpais/2013/12/08/inenglish/1386507145_947657.html

[ancient astronaut]

I am sure there are loads of rats down there as well.