It's a mystery spot inside a museum
From an essay by Luc Sante:
"...a mystery spot is a tilt-induced visual illusion. Its cousin, the mystery hill (or gravity hill, or spook hill), is a naturally occurring phenomenon wherein, as a result of a variety of misleading visual cues, a road that appears to be going uphill is actually going downhill. The mystery spot is a construction of one or more rooms that is significantly tilted off the plumb, frequently at different degrees of tilt per axis, although misleading visual cues are deliberately deployed to conceal this fact. When encountering a rectilinear frame, the human mind naturally tends to want to make it correspond to habitual norms of vertical and horizontal, barring visual indications to the contrary. In a mystery spot the mind, unable to trust in visual anchors, is quite literally thrown off balance..."
Enter the illusion
Step into an optical illusion