Houston is the biggest city in Texas and the fourth largest city in the US. As of the most recent Census, Houston had a population of 2.1 million people within an area of 656.3 square miles making it one of the largest cities by area. The metropolitan area is home to over six million people. Houston is the largest city in the country without formal zoning regulations which makes for interesting real estate. Rather than a single central business district as the center of the city's employment, there are multiple areas throughout the city in addition to Downtown including the Energy Corridor, Galleria / Uptown, Texas Medical Center, Memorial City, Greenway Plaza, Westchase, Midtown and Greenspoint. Some even say Houston has two downtowns due to the size of the medical center in Houston. The medical center is over 1000 acres and by itself would be larger than downtown Dallas. Houston is recognized worldwide for its energy industry—oil and natural gas especially—as well as for biomedical research and aeronautics (NASA). Houston has the third tallest skyline in North America and one of the top 10 tallest in the world. The Houston tunnel system is a seven mile system of tunnels and skywalks that connects many downtown buildings and contains shops and restaurants, enabling pedestrians to avoid summer heat and rain while walking between buildings. Downtown Houston consisted of a collection of mid-rise office buildings. The energy industry boom of the 70's led to a many skyscrapers being erected throughout the 1970s—many by real estate developer Gerald D. Hines—culminating in 1982 with JPMorgan Chase Tower--formerly the Texas Commerce Tower--Houston's tallest skyscraper which has 75 floors and reaches 1,002-foot into the sky. The tower is the tallest structure in Texas and the 10th tallest building in America. Wells Fargo Plaza (originally Allied Bank Plaza) was built the following year. Clocking in at 71 floors, the 992-foot tall Wells Fargo Plaza is still the second tallest building in Houston and Texas. Downtown Houston is largest submarket in Houston and has over 43 million square feet of office space.