Reverse gravity:

I’ve been discussing my adventures biking to work on a rural road. On the way TO work, I would quickly crest a hill, and then zoom down a long sharp decline. On the way back, I would painfully bike up the sharp hill, crest it and then coast slowly for half a mile.
The first few times I tried to coast after cresting the hill were very frustrating. My bicycle simply coasted to a stop. Gradually I learned that on the way home, I could start coasting much sooner than I thought. To put it simply, there was a piece of road that SEEMED to be angled down towards my company, but was actually angled down towards home. Which brings us to the truly interesting question:

How do you KNOW that ground goes up or down? Is it possible to be confused about what’s level?

Years later, a Scotsman told me there’s a famous hill in Scotland where gravity runs in an unexpected direction, basically the same phenomenon. How DOES it work? Could trees be an important factor (if they're not straight, but all angled the same way)?