Even though we humans can only perceive three dimensions of space and one dimension of time, Itzhak Bars thinks there may be two dimensions that we're missing.
"There isn't just one dimension of time," Itzhak Bars of the University of Southern California in Los Angeles tells New Scientist. "There are two. One whole dimension of time and another of space have until now gone entirely unnoticed by us." ...
Changing our picture of time from a line to a plane (one to two dimensions) means that the path between the past and future could loop back on itself, allowing you to travel back and forwards in time and allowing the famous grandfather paradox, where you could go back and kill your grandfather before your mother was born, thereby preventing your own birth.
He has proposed some experiments for testing his theory with the Large Hadron Collider when it opens this year.
It's fascinating to consider how humanity might appear to a creature capable of perceiving all six dimensions described by Bars. I highly recommend the novella Flatland: A Romance of Many Dimensions to get a flavor for such speculation. (Having been written in 1884, Flatland is available free from Project Gutenberg.)