by Keiji » Mon Mar 14, 2011 4:26 pm
Yep, this is a good one.
More formally, in 3D, where we have our vertical, frontal and lateral dimensions (one of each), the frontal dimension is the direction the tunnel goes in, while the sides of the tunnel are positioned laterally and "hold the ground up" vertically. In 4D, there are two lateral dimensions - so one (combined with the vertical) can be used for the sides of the tunnel, and the other (combined with the frontal) can be used to make vast open spaces.