Toronto has a lot of very tall office buildings. Back before TV came via satellite or cable, it was broadcast only through signals sent through the air by transmission towers. The tall buildings made it very difficult for people living in the city to receive TV signals. The solution was to build a really tall TV transmission tower. The CN Tower may be Canada's most recognizable symbol. At a height of 553 metres (over half a kilometre), it is still one of the tallest buildings in the world. There is an outside observation deck at a height of 342 m. Just above that, at 351 metres, is a restaurant that rotates once every 72 minutes, allowing an unobstructed view of the city below. Higher still at the 447 metre level is the Sky Pod, at the time the world's highest public observation deck. The tower is visited by over 2 million people each year. Construction began in February 1973. Almost 50,000 tonnes of earth had to be removed first, in order to create a foundation 7 metres deep that was filled with some 9,000 cubic metres of concrete. Building the foundation alone took over 4 months. The tower was built by continuously pouring concrete; huge steel and wood brackets to contain the concrete were continuously jacked higher as the concrete below hardened. The tower was topped by an observation deck made from steel and concrete. It took nearly 40 months for the 1,537 construction workers, working 24 hours per day for five days a week, to complete the CN Tower. The tower required just over 50,000 cubic metres of concrete, 450 tonnes of reinforcing steel, and 36 tonnes of steel tensioning cables, all of which account for the tower's final weight of over 100,000 tonnes. As the tower construction neared the top, huge segments were ferried upwards by a Sikorsky S-64 helicopter. Heavy precision lift flying of this type takes highly experienced pilots with thousands of hours on the aircraft, working with maintenance and ground crews who are experts at configuring loads and keeping the aircraft flying. I and my friends were able to watch the progress of the construction day-by-day from our residence rooms at the University of Toronto. The CN Tower is not only one of the world's tallest buildings; it also held the record (2013) for the world's highest wine cellar (in the restaurant) and the longest metal staircase. The tower was named one of the 'Seven Wonders of the Modern World' by the American Society of Civil Engineers in 1995. The tower's microwave receivers are located at a height of 338 m above the ground in the radome, the donut-shaped collar at the base of the SkyPod. The equipment supporting the tower's role as a broadcast transmission centre is located here. Further up, at 360 m, are FM transmitters serving many of the TV and radio stations in Toronto. The CN Tower has a glass floor on the lowest observation deck; it's 23.8 square meters of solid glass that is five times stronger than a commercial floor. It can withstand the weight of 14 large hippos. Other CN Tower Facts: The tower leans about 2.5 centimetres at the top because of the Earth's rotation. The tower is hollow. The elevators and stairs are on the outside (but they are enclosed behind glass). There are six glass-fronted, high-speed elevators. They travel at 22 km/h. It takes only 58 seconds to reach the 346 m level. Horizontal bands in the concrete can be seen all the way up the tower. These mark the spots where concrete pouring stopped every weekend. The signatures of 20,000 people are on the antenna at the top. The tower was designed to withstand the impact of a large airplane colliding with it. The tower is designed to flex in high winds. The main pod can move up to 23 centimetres in each direction from vertical. The most recent addition to the site is the ability to hang off the roof of the Skypod and walk back and forth. Visit the CN Tower website at  http://www.cntower.ca Resources HTML, graphics & design by Bill Willis 2023