Top secret(s): the truths behind 10 of the world's great wonders

Ever visit an incredible site in some exotic land and wonder how it was built? The notice board just lists a few meaningless dates, or a guide rattles off a bunch of jargon-packed phrases, but you want the real dirt. Here we reveal the ingenious engineering behind 10 of the world's most epic structures – and the little-known facts that lie hidden in their depths.

Taj Mahal, Agra, India

The Taj Mahal is widely considered one of the most beautiful buildings in the world. Image by Tetra Images - Bryan Mullennix / Brand X Pictures / Getty Images.
Legend has it that Mughal ruler Shah Jahan ordered the hands of the Taj Mahal builders to be chopped off after it was completed, to prevent them from ever building anything so beautiful again - although no written evidence supports this story. The elegance of the mausoleum can be attributed to clever engineering. To make the Taj Mahal appear perfectly straight from ground level, the architect designed the minarets to slant slightly outward, which also ensured that in the event of an earthquake they would fall away from the mausoleum's precious dome.
Make the trip: get to Agra by train from Delhi (about 2 hours). Entry numbers are limited, so buy tickets the day before at the Archaeological Survey India office, known as the Taj Mahal Office by rickshaw drivers (www.asi.nic.in). The Taj Mahal is closed on Fridays.

Burj Khalifa, Dubai, UAE

The Burj Khalifa is a ground-breaking feat of architecture and engineering. Image by Merten Snijders / Lonely Planet Images / Getty Images.
This ethereal tower in the Arabian Desert cost US$1.5 billion to construct. At 828m, 2.5 times higher than the Eiffel Tower, the Burj Khalifa is the world's tallest building. To withstand high winds and earthquakes, this superscraper is designed with a 'buttressed core' – three wings set at 120 degrees to each other, anchored around a central hub. Each wing supports the others, so when the wind blows on two of the wings, the third resists the force.

Moai, Easter Island (Rapa Nui), Polynesia

The moai are one of Rapa Nui's most enduring images. Image by Volanthevist / Flickr / Getty Images.
These ancient statues (among other theories) have been blamed for the demise of the Easter islanders. Transporting them on logs would have devastated forests, and without trees the soil would have washed away, causing failed harvests, famine, war and cannibalism. But satellite images of Easter Island, taken in 2005, show dirt tracks radiating from the quarry where these mysterious 10m-tall statues were carved. After attaching ropes to the head of the moai, small teams could have moved the statues by 'walking' them along.

Stonehenge, England

Aliens, druids and everyone in between have been proposed as the builders of Stonehenge. But why was it built? Recent evidence suggests it was constructed to celebrate midwinter, not midsummer as previously thought. Most of the monuments in the area are aligned on sunrise and sunset at midwinter and, by dating pig teeth found at nearby settlements, it's now known that more pork was eaten then to celebrate days getting longer.
And how was it built? Around 2600BC, bluestones were (most likely) floated on river rafts from the Preseli Hills in west Wales. Radioactive dating proves glaciers couldn’t have swept them to Salisbury Plain 40,000 years ago, as once thought. On site, the foot of each stone was levered into a pit, and lintels lifted into place using scaffolding.
Make the trip: drive to Stonehenge from London (under 2 hours) or take the bus from Salisbury (about 40 mins).

Eiffel Tower, Paris, France

Built for the 1889 World's Fair, Monsieur Eiffel himself was the first to climb the tower’s 1710 steps to the summit. (Having funded most of the construction, he raked in US$1 million in ticket sales in the first year alone.) At 324m, the Eiffel Tower was a useful radio antenna from which the Germans sent coded signals to their forces during WWII. Today, 50 tons of paint are used to resurface the tower every seven years.