The Great Wall of China

In the 220 BCE, the first Emperor of China, Qin (say chin) ordered a wall to be built to join up small forts along what was then the northern border of China as a defence against attacks from the north. Construction was continued by emperors who followed, until the 17th century AD. It is more than 20 000 km long, snaking along hills and valleys, consisting of watch towers, fortresses, horse tracks and shelters along the wall. Its thickness ranges from 4.5m to 9m and is about 7.5 m high. It is considered to be a masterpiece of design and construction, and demonstrates how construction techniques developed and changed during the centuries of construction. Much of the earliest sections are now ruined, but the later sections are open to the public to visit.

The ancient rock city of Petra, in Jordan

This ornate ancient city was carved into red desert cliffs. Around 2,000 years ago Petra was an important centre of trade between Arabia, Mesopotamia, Egypt and countries around the Mediterranean Sea. Temples were carved into the cliffs and houses were built of stone. Protective walls were built on the north and south side of a riverbed or wadi that occasionally has water but is mostly dry. The city had plumbing that brought water into the houses. A series of earthquakes caused the people to leave the city and it lay forgotten for centuries until rediscovered in 1812.

The statue of Christ the Redeemer in Brazil

The first suggestion of building a religious statue overlooking the city of Rio de Janeiro was made in the 1850s, but it was rejected. After a petition in the 1920s, it was agreed to and a design chosen. Construction of this huge statue began in 1922, and took nine years to build. It is almost 30m tall with the 8m tall pedestal it stands on. The arms stretch out to 28m, pointing north and south. The statue was constructed in pieces that were carried up the mountain. In 2003, escalators and walkways were installed so that it is now possible to get onto the platform the statue stands on.

The mountaintop city of Machu Picchu in Peru

The 15th Century city is 2,430m above sea level in the mountains of Peru, on the eastern slope of the Andes. It was built by ancient people, the Incas, and covers 32,500 hectares, and is almost intact. The buildings were made of granite stones that were very precisely cut, and there was no mortar between them. Archaeologists think it may have been a royal estate where the emperor and his family could get away for breaks. There would have been people living there year round to care for and maintain the royal residences. Terraced fields at the city's edge show where crops were grown. The emperor had a residence of his own, separate from other royal residences, with a garden and private bath and toilet - the only private one there. A building known as the Temple of the Sun is near the Emperor's residence. It contains a rock that seems to have been an altar. At the June solstice, the rising sun shines directly onto the rock. The purpose of a giant rock on a raised platform has proved to be a mystery. Scientists think it may have been used to make astronomical observations.

The Chichen Itza pyramid in Mexico

Chichen Itza was an ancient temple-city built around 600 AD by an ancient civilisation, the Maya. The Maya observed and accurately mapped the movements of the sun, moon, planets and stars, and used this information as part of their religion and mythology. They developed an amazing mathematical system and a series of three precise calendars. The pyramid in the Chichen Itza ruins is known as El Castillo, and has four sides, each containing 365 steps, representing a year. There are 52 panels, representing each year of the Mayan century, as well as each week in a year. There are 18 terraces, representing the 18 months of a religious year. Inside, there is a temple reached by a narrow stairway. The city was built near two large, natural sink holes (called cenotes) that supplied plenty of water year round. They were regarded as sacred wells.

The Colosseum in Rome

Built more than 2,000 years ago, the Colusseum is a huge, oval stadium, 188m long and 156m wide, four levels high. Originally known as the Flavian Amphitheatre, building was begun in the time of Emperor Vespasian in 72 AD. It was completed eight years later by his son, Emperor Titus. It had 80 arched entrances to allow entrance of up to 55,000 spectators, who were seated according to their importance. The purpose of the building was the staging for the public of deadly fights between gladiators and wild animals.

Thousands of wild animals and humans died in these contests. Gladiators were mostly men, usually slaves, prisoners of war or imprisoned criminals. A day's event would be a series of fights, watched by a crowd that included the Emperor himself. If the arena became too bloody, a layer of fresh sand was added.

The Taj Mahal in India

The white marble building houses the tomb of Mumtaz Mahal, third wife of Emperor Shah Jahan, and was constructed from 1632 to 1648. Other buildings were added later, finishing in 1653. It is located near in city of Agra. The name Taj Mahal means 'crown of palaces'. A large white dome is the highest point of the building, 171m, surrounded by four smaller domes. The Taj Mahal complex of buildings includes a mosque, a garden, a reflecting pool and other mausoleums (buildings that contain a tomb or tombs). Millions of visitors each year visit the Taj Mahal.

Learn more about the wonders of the world:

• In addition, the Great Pyramid of Giza was made an 'honorary wonder', the only one of the original seven wonders listed over 2,000 years ago that is still standing.
• In 2007, a list of the seven wonders of the natural world was made. Go here to see what they are.