When the Willis Tower (still called the Sears Tower by some of the more intransigent Chicagoans) was completed back in 1974 it was known around the world as the tallest building on the planet. A title it held for almost 25 years. Since that time, however, the Willis Tower has seen building after building surpass it in height. So many buildings, in fact, that it is no longer even in the top 10.