Is mysterious city 'spotted floating in clouds above China' a message from an alien race? മേഘങ്ങള്ക്കിടയില് കണ്ട നിഗൂഢ നഗരത്തിന്റെ രഹസ്യമെന്ത്? Scientists believe it is an optical illusion - but what is the truth behind the ghostly city that appeared in the clouds?