The CN Tower turns 40 on Sunday, marking a milestone for Canada’s tallest and arguably most iconic building.

Opened in Toronto in 1976, the CN Tower was the world’s tallest free-standing structure when it was built. Its record was later surpassed by several structures (the Burj Khalifa tower in Dubai is the current record-holder), but Canada’s 553-metre tower remains the tallest tower in the Western hemisphere.

CN Tower construction - 1975

Aside from its prominent position in city skyline, the CN Tower serves an important telecommunications role. More than 16 TV and FM radio stations broadcast signals from the tower’s antenna, and cell phone signals also use the tower.

More than 1,500 construction workers began building the structure in 1973 and worked around the clock for 40 months to finish the project.

CN Tower

But the construction was not without drama. In February 1974, a helicopter became stuck to the tower’s peak as it attempted to dismantle a crane at the top. With less than an hour’s worth of fuel left for the 12-minute job, the chopper needed to be detached immediately. To make matters trickier, the crane’s operator was still inside.

A construction crew quickly shimmied up the tower and burned off support bolts that had seized and caused the problem. The helicopter and crane were finally released from the tower and made a safe landing nearby, with about 14 minutes of fuel left in the tank.

CN Tower

The tower officially opened on June 26, 1976, and its inauguration was a major celebration. Politicians from across the country came to celebrate the opening, including then-Prime Minister Pierre Trudeau. Visitors arrived in bell-bottom jeans and wide-collared shirts to take in the brand new panoramic views of the city.

A time capsule is securely locked inside the tower’s look-out level, with plans to crack it open on its centennial anniversary in 2076. The capsule contains letters written by various school children, copies of three daily newspapers and a never-before-seen letter from the elder Trudeau, who sealed the capsule during the tower’s grand opening.

CN Tower

Today the CN Tower acts as a bellwether for significant global events and Canadian celebrations. The tower was recently lit up in royal purple to celebrate the Queen’s 90th birthday, which happened to coincide with the death of “Purple Rain” singer Prince. Next week, the tower will go red and white for Canada Day and boast the colours of the rainbow for Sunday’s Toronto Pride parade.

Toronto CN Tower Earth Hour

The CN Tower takes its name from the Canadian National railway company, but the company was divested from the tower in the mid-1990s.

In 1995, the CN Tower was named one of the Seven Wonders of the Modern World by the American Society of Civil Engineers. The tower shares the designation with the Empire State Building, the Golden Gate Bridge, the Channel Tunnel, the Itaipu Dam, Europe’s North Sea Protection Works and the Panama Canal.

While it isn’t the tallest building in the world anymore, the CN Tower holds on to a couple other Guinness World Records, including the world’s highest wine cellar (located in the rotating 360 Restaurant) and the highest external walk on a building, the EdgeWalk.

"So much of the time we're looking up at all these towers and feel dwarfed by them," Jaspal Atwal, speaking before he did the EdgeWalk with his wife Kalbinder on Sunday, said. " So we get on top of the city and get to look down."

"It's only when you look down, that's when the adrenaline kicks in," Kalbinder added after the adrenaline pumping walk.

Cn tower edgewalk