Colby Cosh nominates the ATM, and he has a point. But for the #1 underrated invention of the 20th century, I’m gonna have to go with air conditioning, invented in 1902 by Willis Carrier. On enormous swatches of the globe it is just too damn hot to work, even to consider working, for four to six months of the year. It’s impossible to imagine Houston, or Dallas, or Miami as they are today without air conditioning. Colby guesses that ATMs save 5-10 hours of time in line per person per year, which is a lot. But to double or triple the productivity of labor for a big chunk of the year at every latitude south of the Mason-Dixon line — what’s that worth?