The city of Mecca is an interesting place. Because of its position in a narrow valley, they’ve had to work hard to continue to expand the transportation system over the years.
As a result, many tunnels have been built under the city to handle both pedestrians and vehicles. While you can can see many of the tunnel entrances in Google Earth, it can be difficult to determine where they all lead. A handful of users in the Google Earth Community (led by “Diane9247″, with help from various others) have been mapping out the tunnels to try to determine where they all lead. The thread is an interesting read, and you can view the results of their work in this KMZ file (shown below).
During the annual pilgrimage to Mecca, where roughly three million pilgrims travel to the city, the capacity of the tunnels becomes very tight. Over the years there have been a number of tragic incidents, typically caused by a stampede of people. This file really showcases just how immense the network of tunnels has become.
[UPDATE: As some of you pointed out, between the time this mapping started and now, updates to the Google Earth road layer made much of this work obsolete. However, it's still a nice testament to their hard work, and they point out a few additional areas that road data doesn't cover. It'll be real neat once StreetView covers the area!]