(This is one of Seven Answers).
Deep water. It's not the fear of having nothing beneath you, but the fear of having EVERYTHING beneath you, and the fear of the totally unknown. I had a discussion about this with my best friend when I was about 17. She agreed with me. She thought about it for a while and then described how she felt about it. She told me to look at the ceiling. We were in the sixth-form common room at the time. I looked up. Then she said to imagine I could see just the legs of an Action Man doll, protruding from the ceiling. I pictured them, very small and a long way up. Then she looked around the room. She said, 'Now imagine all this room is full of water and it's really dark down here - think about being that action man.' We both shuddered. Not everybody gets it, and that's fine, but I still have trouble with ferry crossings and I can't ever imagine myself on a cruise liner.