Mitr F.
Google
To begin with, it doesn't feel too fascinating, when you're associating it with UNESCO! However when you understand that this was built in 1925-31, in the beginning of industrial revolution, when all other buildings were built of bricks and primarily opaque - this, primarily made of glass & steel and primarily transparent - is an absolute shocker! This paved way for, set an inspiration for all the sky-scraper office buildings that you see today that are just glass & steel!