Houston Tunnels

The Houston tunnel system is a network of subterranean, climate-controlled, pedestrian walkways that links 95 full city blocks 20 feet below Houston's downtown streets. It is approximately six miles long. There are similar systems in Chicago, Dallas, Oklahoma City, Montreal, and Toronto. Architectural historian Stephen Fox has stated that the idea for the tunnel system came when the Bank of the Southwest Building was "linked by tunnel to the 1010 Garage and the Mellie Esperson Building" in 1961.

Source: Wikipedia