The Sears Tower under construction in Chicago, circa 1973.
The site of Sears Tower is cleared as construction begins.
The Sears Tower, right, begins to take shape in downtown Chicago in the early 1970s.
Workers take in the view from the 80th floor of the Sears Tower under construction on November 6, 1972.
The skeleton of Sears Tower, seen from across the street at Wacker and Jackson streets on September 21, 1971.
Sears Tower became the tallest building in Chicago when construction continued on November 6, 1972.
Sears Tower became the tallest building in Chicago, surpassing the John Hancock Center seen in the background, when construction continued on Nov. 6, 1972.
Sears Tower under construction on April 24, 1972.
An ironworker navigates the Sears Tower as the building nears completion on April 13, 1973.
The phases of construction of the Sears Tower are seen from the intersection of Morgan and Harrison streets, near the University of Illinois Circle Campus on Nov. 24, 1972.
Ironworkers work on an upper level of the Sears Tower under construction on April 13, 1973.
Passersby watch construction begin at the site of the future Sears Tower on June 3, 1971, in downtown Chicago.
Perched high on the Sears Tower, workmen go about the task of constructing the world’s tallest building on April 13, 1973.
A view of the Sears Tower in 2002 from Roosevelt Road where a small patch of prairie grows between the railroad tracks and downtown.
Security guard Todd Marshall, on the rooftop of the Sears Tower in 2002, checks the building from top to bottom every night.
Security officer Todd Marshall worked from 3 to 11 p.m. at Sears Tower in 2002. His daily routine included inspecting the skyscraper from the rooftop to the loading docks to the bottom floor of the garage.
High atop the Sears Tower in the Skydeck, a computer monitor displays a photo of the building as the city lights spread out to the south in 2002. On the monitor the moon shines over the Sears Tower while in live time it rises off to the east.
A view of the Sears Tower from the Kinzie Street bridge along the Chicago River in 2002.
The Sears Towers looms in the background as marathoners run west on Adams Street in Chicago in 2003.
In 2007, the Sears Tower was still searching for tenants to fill vacant space.
The Sears Tower rises above the skyline in a view from the Adler Planetarium on a below-zero day in 2009.
Journalists check out “The Ledge,” a new addition to the Sears Tower, in July 2009. The enclosed glass ledge juts out from the 103rd floor of Sears Tower, which at the time was the tallest building in the United States.
New signage adorns the newly named Willis Tower in 2009.
Lighting strikes both the Willis Tower and the Trump Tower in downtown Chicago as a severe storm rolls through the region in June 2010.
United Airlines announced in 2010 that its new state-of-the-art operations would be centered in the Willis Tower.
The Willis Tower, the moon and pink clouds make for a beautiful sunset in Chicago in 2011.
The Willis Tower is bathed in clouds before a 2012 spring storm in Chicago.
Willis Tower and the Chicago skyline on Nov. 12, 2013, the day the tall buildings council announced that One World Trade Center would become the country’s tallest building when completed.
The sun rises behind Willis Tower and the city of Chicago on August 15, 2013.
Adil Khan, Zubair Iftekhar and Aamer Khan take photos Feb. 4, 2015, on the Ledge at Willis Tower. The Khans were visiting from India and Iftekhar is a student at UIC.
The Willis Tower peeks through heavy fog and rain clouds early on June 14, 2015.
Willis Tower looms large as seen from the Merchandise Mart on June 1 2015.
Willis Tower is visible through the most iconic element of the new 606 trail — a rebuilt bridge over Milwaukee Avenue, painted dark red like the historic bridges over the Chicago River.
Steam comes off of the Willis Tower after morning showers on May 6, 2010.
The Shedd Aquarium frames Chicago’s Willis Tower, whose antennas are lit in red in support of the Chicago Blackhawks, set to play Game 1 of the Stanley Cup Final on June 3, 2015.
Willis Tower is framed in the columns of Union Station at sunset in Chicago on September 25, 2014.
Chicago’s Willis Tower looms ahead as the Wienermobile gets stuck in traffic with other vehicles on June 21, 2015.
Willis Tower, the world’s tallest for many years, looms over the Chicago River’s South Branch.
The Willis Tower was deemed the second-tallest building in the U.S. after the Council on Tall Buildings and Urban Habitat announced that New York City’s One World Trade Center building would be the tallest building in the United States upon completion and occupancy.
