Location:  The CN Tower is located in Toronto, Ontario, Canada. The Tower was built beside Lake Ontario.

Description:

The CN Tower in Toronto, Ontario, is the largest free-standing structure in the world. It is an observation and communications tower and stands 553 m (1815 ft) tall. Over 62,000 tons of rock were removed to install a 6.7 meter thick concrete and steel base using more than 7,000 cubic meters of concrete, 500 tons of reinforcing steel and 40 tons of tensioning cable.

The three legs are hollow, and the middle core is empty. One of the main reasons it was built hollow was to ensure flexibility. It was also a great way to install all the water pipes and electrical cables without sticking them on the outside.

history:

In 1972, extensive soil testing was used to calculate the effect of the Tower on the bedrock under Toronto. With these results, the engineers were able to design a foundation that would ensure the Tower's stability.  After the extensive planning stage, concrete was poured 24 hours a day, 5 days a week into a massive 'slipform' which inched its way up the Tower for an entire year.  As the concrete hardened, a ring of hydraulic climbing jacks moved the form upwards, gradually decreasing in size to create the gentle tapered contour we see today. The CN Tower was completed in 1976.

Interesting Facts:

In spite of all this planning, the Tower still leans. But don't worry, it leans less than an inch. This "lean" is due to the earth's spin combined the Tower's height.

Today, if you look closely at the Tower exterior, you'll notice horizontal lines at various intervals. The builders call these 'Friday lines', because they mark where the pouring of the concrete was halted each weekend.

In 1976, before the last antenna was lifted to the top of the tower, everyone was invited to visit the construction site and sign the antenna. About 20,000 people did just that- crawling inside, scribbling their names, leaving their mark on history.