2. Istanbul's Hagia Sophia

Fiery Roman Emperor Justinian ordered the rebuilding of the great church of Hagia Sophia in Istanbul almost 1500 years ago. Modern science now uncovers the secret to this vast golden dome's longevity.

3. Petra

Carved into the side of a narrow gorge in the unforgiving deserts of Petra in modern day Jordan, the ancient Nabataeans turned engineering on its head to create their glorious royal city, Khazneh.

4. St. Paul's Cathedral

From the ashes of London's Great Fire in 1666 rises Christopher Wren's daring architectural vision. As England's first Baroque cathedral, St Paul's is an icon to rival the grandeur of ancient Rome.

5. Angkor Wat

Hidden deep in Cambodia's jungles, the temple of Angkor Wat is a towering monument to the Hindu god Vishnu, belying its origins as the mausoleum for a usurper king who murdered his great uncle.

6. Machu Picchu

Built using techniques that surpass modern ones, the temples, altars and residences of Machu Picchu have resisted torrential rains, landslides and earth tremors since being abandoned by the Incas.