Mine = Swimming in open waters.
When I was a boy, around the age of 6, my father took me swimming with his friends in a small lake where I grew up. My father gave me an inner tube to cling to and told me to hold on to the inner tube and I'd be safe. I couldn't yet swim.
I vividly recall losing grip of the tube and sinking below. I was only underwater for a few moments. The helplessness was a horrible feeling, I gulped in some water while frantically reaching upward to grip what was no longer there. In less than a minute, my dad pulled me up, out of the water an placed me on the tube. I was fine, but that moment has never left me. To this day I can't enjoy swimming the way others do. My wife loves it, many people love it but I still have a slight fear when swimming in open waters. Jumping off of a dock or swimming out from shore is fine, but when I'm out in the middle of a big lake or a body of water I still get uneasy.
That's mine. What is yours?