The operators of the small site (which is about 150 feet in diameter) claim that it is a place where the laws of physics and gravity do not apply and provide a number of demonstrations in support of these claims, where water seems to flow upwards, people seem to be standing in slanted positions etc.
Illusion explanation 
The Mystery Spot is a gravity hill, a tilt-induced visual illusion. The illusion experienced by visitors results from the oddly tilted environment as well as standing on a tilted floor. Inside the tilted room of the Mystery Spot, misperceptions of the height and orientation of objects occur.
Even when people are standing outside on a level ground, the slant of the building in the background causes misperceptions as we judge the height of people using the slant of the roof rather than the true horizon.
- Shimamura, A. P. & Prinzmetal, W. (1999). "The Mystery Spot Illusion andts Relation to Other Visual Illusions". Psychological Science 10 (6): 501–507. doi:[http://dx.doi.org/10.1111%2F1467-9280.00196 10.1111/1467-9280.00196.]
- Mystery Spot official website (California)
- St. Ignace Mystery Spot (Michigan)
- Video Tour of the Mystery Spot (California)
- Mystery Spot explained (SandlotScience.com) - this site contains explanation about how Mystery Spots work, including a list of mystery spots
- Further article explaining the illusion (Berkeley.edu)