For me, it's the fact that that entire space could be filled with water.
I used to get the same eerie feeling lifeguarding as a kid, walking around in drained Olympic size pools. Always weirded me out for a minute. Like any second it could fill back up.
Yes, I would be 3x as scared if this was full of water, I was floating on top of it, and knew that all of this was below me. (and couldn't see to the bottom) It's why I am scared of dams.