I once had a chance to set foot inside anechoic chamber designed for testing the sonic characteristics of speaker design. It was so quiet--and I'm not making this up--that I could hear the sound of blood being pumped through my body. I shit thee not. It was terribly surreal.
Aneochoic chambers are important for all sorts of scientific and market research. The problem is that almost no one can stand being inside the chamber for too long. The problem is hallucinations. The quiet is so disorienting that your brain can't cope.
There's a very good anechoic chamber in Ottawa, but the very best in the world--according to the Guinness people, anyway--is in Minneapolis which reduces reflected sound by at least 99.99%. That's beyond dead quiet.
Read more here from the Daily Mail. That's where the picture is from, too.