In times where I'm looking at this sort of stuff, I'm reminded of a road in Mexico where me and my cousin stopped for a bit and I saw a natural (?) optical (?) illusion. Driving between cities can be tiring in those parts so I thought maybe my cousin just needed to stretch out a bit. However he told me to get out of the car as well and come take a look at something...
I see him standing in the middle of this road and he points in one direction. "What's that way? Uphill or downhill?" I look in that direction and say "Uphill." He says "Right that's what we both see. And if we look back over there, we can both see that's downhill."
It was a long day and I was tired. I was getting a little annoyed by whatever this little charade was, but I looked in that opposite direction and said "Yep, obviously that's downhill. WTF you getting at?" Then he shows me a bottle of water and puts it on the ground carefully and lets it roll on its own... WUT? I did a double take at the incline... Fucking bottle rolls uphill. Or rather what he and I perceive to be uphill.
We then get in the car and pull it into the road. He puts in in neutral and it also starts to coast up the hill.
I've looked it up on the internet, but I think my google-fu is too weak to find an explanation. I've simply accepted it as a real lesson in how feeble human perception is. It's called "La Zona Magica (Magic Zone)" There's tons of video on Youtube.
Here's one in english.