Standing the test of time
Great Wall of China
From the 7th century BC, Chinese rulers started building walls around their states to keep out their enemies.
When the emperor Shih Huang Ti united China in 221BC, he gave orders that the walls dividing the provinces be knocked down. But he also ordered his troops to build a wall along the northern border to keep out wandering Xiongnu people.
Between 214BC and 200BC hundreds of thousands of workers constructed the 10,000 Li Long Wall. (A li is an ancient Chinese measurement equivalent to about 0.5 kilometres). But when the emperor died, the wall was abandoned.
About 300 years later, the Han emperor Wu Ti started rebuilding the wall. He decreed it would have 'a beacon every five li, a tower every 10 li, a fort every 30 li and a castle every 100 li'.
Over the centuries other rulers added to the construction. Today the Great Wall of China runs 7,300km from the Shanhai Pass in the east to the Chiayu Pass in the west.
The wall is made of stone, brick and in some places of dirt. It's five metres tall at its lowest levels and nine metres at its highest. A four-metre-wide road on top connects the watchtowers. There are also camps for troops behind the wall. The Great Wall is now a World Heritage site. Even though erosion and construction have taken their toll, much of it remains intact and attracts many tourists.
Stonehenge is a circle of huge stones surrounded by a big earth wall, located near the town of Salisbury in England.
Some believe Stonehenge is an ancient place of worship where religious ceremonies took place on important days of the year - like the summer solstice. Human bones, pottery and tools have been found at the site.
What archaeologists do know is that Stonehenge was built over a long period of time.
In 3100BC Neolithic people dug a ditch that was two metres deep and 98m in diameter. The chalk from this was used to build a surrounding wall. Two huge stones were then set up at the entrance to the circle.
In 2100BC two circles made up of huge bluestone rocks brought from Wales some 250 kilometres away were added. How the people dragged the stones that far is uncertain. They also made an entranceway that was aligned with the sunrise on the summer solstice, the longest day of the year. A hundred years later, they added another circle of 30 upright stones weighing 50,000 kilograms each and topped with stone lintels.
There are hundreds of similar stone circles in Britain but Stonehenge is the biggest and most famous. Although many stones have been removed over the centuries, it remains an important archaeological and World Heritage site. One day we may yet discover its purpose.