Willis Tower—OK, many of you still call it Sears Tower—is a beacon of civic pride for Chicagoans everywhere. It's as iconic a symbol of the city as Al Capone, Michael Jordan, pizza and hot dogs. (No ketchup!)

Willis was the world's tallest building until 1998, and the Skydeck is still the tallest observation tower in the US. Although we point at 233 South Wacker as an icon of architecture today, much of Willis Tower's history was fraught with frustration. Here are five fun facts you may not have known about Willis Tower.

1. It Was The Major Corporate Office Consolidation Of Its Time

Sears’ recent history has been marked by plummeting revenues and store closings, but in 1969 it was the world's largest retailer, with over 350,000 workers. Sears decided the thousands of employees scattered throughout the Chicago area should all be relocated to one building, and hired the law firm Arnstein & Lehr (then Arnstein, Gluck, Weitzenfeld & Minow) to consult with federal and local authorities on a location. The choices were narrowed to two: Goose Island and a two-block area bounded by Franklin Street, Wacker Drive, Adams Street and Jackson Boulevard. Sears decided on the second site, bought the 15 buildings that occupied it and paid the city $2.7M for the stretch of Quincy Street that intersected the site.

2. It Was The First Building To Use Bundled Tube Design

Structural engineer Fazlur Rahman Khan jump-started a new era in skyscraper construction with the various tube designs that, to this day, remain the foundation of modern supertall structural engineering. Khan, fresh off the triumph of his tressed tube design for the John Hancock Center, worked with Skidmore, Owings & Merrill architect Bruce Graham to design Willis Tower as nine separate buildings, or “tubes,” that all rose 50 stories. From there, the northwest and southeast tubes end, while the northeast and the southwest tubes rise to the 66th floor. The north, east and south tubes stop at 90 stories, and the center and west tubes rise a full 108 stories. The bundled tube design was structurally efficient and saved money: Willis Tower’s construction cost less money per area than the Empire State Building.

3. Opponents: The Height Will Interfere With Television Reception

This seems trivial in an age of a la carte broadcast media consumption, but several interest groups worked to keep Willis Tower from rising past 67 stories during its construction, over fears the building would interfere with television reception. The Lake County State’s Attorney filed a suit on March 17, 1972, seeking to halt construction, claiming residents had a constitutional right to television reception without interference. A second suit was filed by the suburbs of Skokie, Northbrook and Deerfield on similar grounds. Sears filed motions to dismiss the suits, which were granted in June 1972. Eventually, two television antennas were added to the tower in 1982. The western antenna was extended to improve NBC5’s reception in 2000, bringing Willis Tower’s overall height to 1,730 feet.

4. The Building Had A Vacancy Problem For Its First Decade

Throughout Willis Tower’s construction Sears offered rosy outlooks for the company’s future growth and laid out a strategy where floor space would be leased to other companies until Sears was ready to retake it. But it wasn’t to be. Sears lost market share throughout the ‘70s as retailers like Kmart, Kohl’s and Walmart grew. The building instead became a symbol of shadow space—it was half-vacant until the mid-1980s. Sears began moving out of the building in 1992 and sold the tower to AEW Capital Management two years later.

5. Ownership Changed Hands Like A Hot Potato The Past 20 Years

Willis Tower was 33% vacant when AEW acquired the building. It sold the tower in 1997 to TrizecHahn Corp for $110M, assumption of $4M in liabilities and a $734M mortgage. TrizecHahn surrendered Willis Tower to MetLife in 2003, which sold the building in 2004 to a JV of Joseph Chetrit, Joe Moinan and American Landmark Properties. With this team came stability and marquee tenants like United Airlines in 2012, and Willis Group Holdings, which paid for the naming rights in 2009. The Chetrit-led JV sold Willis Tower to Blackstone Group for $1.3B last year, in the largest American real estate deal outside of New York City.