Edges.
When I was five or six I almost fell down a large pit, probably part of a drainage system on a building site ( my parents’ friend took us to see his new school being built when he probably shouldn’t have). I stepped on a concrete paving stone which cracked and I ended up with one foot trapped and the rest of me in the hole. I remember it being bottomless but probably ten or fifteen feet deep.
Ever since I have hated edges-quay sides, cliffs, bridge parapets etc. etc. As a kid I would walk as far away from any edge as possible but if forced to stand near an edge I would feel strangely drawn to lean out and fall- scared the pants off me for years
Strangely enough I don’t have a fear of heights if I have started from the bottom, such as climbing ladders or masts of boats.
Mountain biking has greatly helped me come to terms with this, but I still am very uncomfortable on some rides with drops to one side