You might think that a library reading room or well-insulated basement is a quiet place, but compared to the anechoic chamber in Microsoft's audio lab, they're about as quiet as a jumbo jet. The chamber, housed in Building 87 on the company's Redmond, Washington, campus, was recently verified by (who else?) Guinness as being the quietest place in the entire world, reaching an eardrum-bending -20.3 decibels.
That's amazingly close to the limit of how quiet things can be on our planet: -23 decibels or so, which is the amount of noise made by air molecules bouncing off one another. Of course, it's much quieter in space — silent, in fact. But going to space is cheating, since there's no air to propagate sound. The previous record was -13 decibels, set at Orfield Laboratories in Minneapolis.
Anechoic chambers are lined with wedge-shaped sound absorbers that take any vibration in the air and dampen it, and at Microsoft, the whole room is separated from the rest of the building so things like footsteps and other vibrations don't leak in. The result is a quietness that feels eerie and unnatural.
Microsoft and others use chambers like this to test equipment for noise levels, to provide a perfectly silent environment for training voice recognition, and so on. You can learn more about sound ratings and the B87 anechoic chamber at Microsoft's website.