Four thousand five hundred years ago, the Egyptians built the pyramids of Giza as grand and impressive tombs for their pharaohs.
The largest of these enormous monuments is almost 147 meters high, and for four thousand years, it was the tallest man-made structure on earth.
Today the tallest man-made structures are skyscrapers.
Every day in cities around the world, millions of people go to work in towers made of steel and glass.
These modern structures exhibit the best in construction technology and engineering skills.
And the modern city could not function without them.
When the demand for living and working space in the limited area of a city's downtown grows, there is nowhere to build but up.
Smaller buildings are demolished, and taller buildings are built to replace them.
The streets of Manhattan in New York City, where real estate for new buildings is limited, are lined with tall, gleaming office towers.
During the first half of the nineteenth century, buildings were rarely taller than five stories.
Buildings higher than this would have been impractical—office workers would have become exhausted from climbing so many stairs!
Then in 1853, Elisha Otis invented the safety elevator.
Previous elevators were used mostly for freight, not people.
If the cable that held the elevator snapped, the elevator would fall, killing those inside.
The safety elevator has mechanical locks that automatically hold the elevator firmly in place if the cable snaps.
The invention of the safety elevator made tall buildings a practical reality.
While Elisha Otis was improving the elevator, other inventors were hard at work improving water pumps.
As cities expanded, more powerful pumps were needed to provide water to their distant areas.
Eventually pumps became powerful enough to supply water to great heights, enabling cities to grow upward as well as outward.
In the middle of the nineteenth century, manufacturers developed the process for mass-producing steel, which is stronger and lighter than iron.
With steel beams, buildings could be built around a metal frame, rather than out of stone walls.
Stone walls are impractical for very tall buildings.
If stone blocks are piled too high one on top of another, their combined weight will crush the bottom blocks.
Steel-frame construction and improvements like the safety elevator and powerful water pumps allowed buildings to soar higher than ever before.
Soon builders were competing to construct taller and taller skyscrapers.
When Walter Chrysler, founder of an automobile company, started work on the Chrysler Building in New York,
he wanted it to be the tallest building in the world.
But during its construction, work began on an even taller building—the Bank of Manhattan Building.
Chrysler, however, had kept something secret: inside his building a giant spire was under construction.
After the Bank of Manhattan Building was finished in 1930, the spire was placed on top of the Chrysler Building.
It extended the building's height to about 319 meters, making it the world's tallest building.
But the competition for the tallest skyscraper did not end there.
A year later the Empire State Building was finished, standing almost 450 meters high.
When it became possible to build skyscrapers hundreds of meters tall, unusual engineering challenges surfaced.
Strong winds can cause a tall building to sway back and forth.
To stop the building from shaking, many skyscrapers have a central core of steel around the main elevator shaft.
This makes the skyscraper more rigid.
Also skyscrapers must use a special type of glass.
Tempered glass, or safety glass, is stronger than ordinary glass.
When safety glass breaks, it shatters into small pieces with rounded edges.
Ordinary glass in a skyscraper window would be hazardous; if it broke, large, sharp pieces would fall and injure people on the sidewalks below.
Tremendous skill is required to build skyscrapers.
The steel beams must be riveted and welded together before the floors and walls can be put in place.
Before workers were required to wear safety harnesses, it was very dangerous work.
One mistake or an unexpected gust of wind could send a construction worker falling to his death.
For much of the twentieth century, the Mohawk Indians were considered some of the best construction workers.
Many of them fearlessly strode along the narrow beams of steel, hundreds of meters off the ground, earning the nickname skywalkers.
New skyscrapers are still being built.
As of 2014, the tallest skyscraper on earth is the Burj Khalifa in Dubai.
It is 828 meters tall and was completed in 2009.
In 2014 plans to construct an even taller skyscraper in Saudi Arabia—the Kingdom Tower—were announced.
If completed, the Kingdom Tower will have three hundred floors and reach one kilometer into the sky!
Thanks to advances in materials and technology, today's skyscrapers stand many times higher than the pyramids.
And architects and engineers keep dreaming about even taller structures.
Future man-made structures will likely reshape the urban landscape—just as the first skyscrapers did over a hundred years ago.