Caves. I went through a spelunking/caving phase over the summer, and if you turn off all the headlamps at once, not only do you get complete silence, but if you're in the right cave, you get complete darkness. Which messes with your mind a bit, I won't lie, but it's also something cool to try.
One night, I decided to swim in our pool alone. While I was submerged, I realized that was the very first time I really couldn't hear anything. Even my own heartbeat.
I once had access to a comppletely soundproof room for practising music, but your nervous system emits a sort of ringing noise; it's always functioning at a specific frequency.