Two things make tall buildings possible: the steel frame and the safety elevator. The elevator, underrated and overlooked, is to the city what paper is to reading and gunpowder is to war. Without the elevator, there would be no verticality, no density, and, without these, none of the urban advantages of energy efficiency, economic productivity, and cultural ferment. The population of the earth would ooze out over its surface, like an oil slick, and we would spend even more time stuck in traffic or on trains, traversing a vast carapace of concrete. And the elevator is energy-efficient—the counterweight does a great deal of the work, and the new systems these days regenerate electricity. The elevator is a hybrid, by design.
From a superb New Yorker essay by Nick Paumgarten on the lives of elevators (a few years old now).
I've become increasingly fascinated with the history of elevators since taking a really bad photo of the elevator (erm, lift) controls at Harrods.