The iron pillar is a 23 foot high, wrought iron standard, erected in honour of Lord Vishnu by King Chandragupta Vikramaditya (Gupta dynasty) almost 2000 years ago. Its claim to fame is its resistance to corrosion despite being open to the elements for so many centuries!
This picture was clicked by me whilst visiting the Qutb-Minar complex and shows the iron pillar (to the left and in front) flanked by the Qutb Minar at the back. The pillar was moved to its current location by the Turk Qutb-id-din Aybak, when he had come on his crusade through the Indus valley into India. He vandalised all the temples and used them as construction material, for his monuments, but surprisingly did not (or was unable to) damage the Iron Pillar.
After scientific analysis, the secret of the miraculous anti corrosive properties of the iron have finally been deciphered. The corrosion resistance results from an even layer of crystalline iron hydrogen phosphate forming on the high phosphorus content iron, which serves to protect it.
The above explanation is from Wikipedia and the entire article can be found here. This is only one of the examples of the amazing building techniques that had been developed by ancient India…other remarkable examples about in the ruins of Orissa and Madhya Pradesh, with a famous example being the Konark Sun Temple. It has massive granite blocks with perfect edges slotted into place without joints or mortar. Add to it, the huge legendary lodestone that used to be present in the temple garbhagriha, and the fantastically detailed carvings adorning the entire complex…all making it a truly wondrous monument (pictures in my next post)