The Houston tunnel system is a network of subterranean, climate-controlled, pedestrian walkways that links 95 full city blocks 20 feet (6 m) below Houston's downtown streets. It is approximately six miles (9.7 km) 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.

Houston is the most populous city in the U.S. state of Texas and in the Southern United States. Located in Southeast Texas near Galveston Bay and the Gulf of Mexico, it is the seat and largest city of Harris County; as well as the principal city of the Greater Houston metropolitan area, which is the fifth-most populous metropolitan statistical area in the United States and the second-most populous in Texas after Dallas–Fort Worth. With a population of 2,302,878 in 2022, Houston is the fourth-most populous city in the United States after New York City, Los Angeles, and Chicago, and the seventh-most populous city in North America. Houston is the southeast anchor of the greater megaregion known as the Texas Triangle.