Iodized salt is so ubiquitous that we barely notice it. Few people know why it even exists. Iodine deficiency remains the world's leading cause of preventable mental retardation. According to a new study, its introduction in America in 1924 had an effect so profound that it raised the country's IQ.

A new NBER working paper from James Feyrer, Dimitra Politi, and David N. Weil finds that the population in iodine-deficient areas saw IQs rise by a full standard deviation, which is 15 points, after iodized salt was introduced.

The key to the level of silence is the fact that the walls, floor and ceiling absorb all sound, rather than reflecting it, as most surfaces do. Thus the term anechoic: no echo. It’s so quiet, you can hear your own organs: your heart, stomach, even your ears, which make a tiny amount of noise. It turns out that it’s not an especially pleasant experience, especially in the dark. The longest anyone has ever spent alone in the chamber? Forty-five minutes.