What's the most bone-chilling sound you've heard in nature?

It's a tie, for me, between a tornado and a pack of wolves howling.

1. The tornado was approaching our home, where we had taken cover in the hallway. It destroyed our home and killed my neighbors.

2. I was at the gate of a national park at 5am, waiting for the park to open. It was still completely dark outside, and I was sitting at a nearby picnic table with some other people. The wolves were very close when they started up.