Houston, Texas also has an extensive underground tunnel system connecting large parts of the downtown business district and government buildings. It's a godsend during hot, swampy summers when commuting by foot at street level guarantees being soaked in sweat within a block or two, and is probably the most popular spot to head for lunch in the downtown area.
One interesting factor here is that aside from the portions serving government buildings, the tunnel system has grown organically as each piece is constructed and maintained by the owner of the building above. Direct tunnel access from a building is considered a major perk for employees and drives up commercial rent prices in each section's associated building.