Anechoic chambers are told to be the quietest places, used mainly for experiments and measurements. If you hold your breath, you can hear the footsteps of a centipede.
This article suggests they are a serious challenge to endure. According to Steve Orfield, founder of Ofrield Laboratories, some people find it very relaxing, while others found it fearful and claustrophobic. This guy proves it’s not that bad. If only he had used a proper microphone, so that we can hear the actual nothing.