I have a fear of heights and the deep water. Why?
Because I know that under me are hundreds to millions of individual thinking minds... Scary thought. I haven't even mentioned the fear of becoming stranded with my two little sisters for company. VERY scary, indeed.
I can deal with being on the lake, though I can't go in the water. I know I can swim. The thought of NOTHING being under me to stop me from drowning is the thought that scares me.
I have never been higher off the ground than I can jump (except for those few times I have jumped off swings). This makes flying even more scary.
As for the land, there are millions of insects under me this very second. The only good thing: I can't fall through the Earth or drown in the dirt.