Our sense of gravity/balance is not very precise on its own and it is combined in the brain with visual clues and other information. For example, it is really difficult to hang a picture straight on a large wall if you don't have a level tool or can align it parallel to the floor/ceiling. There are a few effects that can distort your sense of what is "horizontal".
The Ames room is an optical illusion where you think lines to be perpendicular where in reality they aren't. If the surrounding hills, trees etc. form lines at certain angles (e.g. a ridge on the horizon that your brain expects to be horizontal but that really slopes), you can get a similar effect in nature. Wikipedia explains it in relation to "magnetic mountains" or "gravity hills":
Ames' original design also contained a groove that was positioned such that a ball in it appears to roll uphill, against gravity. Richard Gregory regards this apparent "anti-gravity" effect as more amazing than the apparent size changes, although today it is often not shown when an Ames room is exhibited.
He speculates that "magnetic hills" (also known as "gravity hills") can be explained by this principle. For a magic mountain at an unnamed location in Scotland, he found that a row of trees form a background similar to the setting of an Ames room, making the water in a creek appear to flow uphill.
Also, your brain adjusts the sense of balance over time. You often get this when you cycle up a long steep hill followed by a less steep road with very little inclination. The flatter road may look like actually going downhills (even though it's still uphills) because on the steep climb your brain adjusts and your sense of what is "horizontal" gets tilted a bit. Often, its only when you turn around that you actually get a real sense of the inclination.
