An unsolved mystery, the IRON PILLAR OF DELHI now standing at Quwwatul mosque at Mehrauli in Delhi, India. The 7.21 meters tall structure all most 1600-year-old stands completely rust-free. The pillar was constructed by “King Chandra” probably Chandragupta2

pillar was certainly used as a trophy in building the Quwwat-ul-Islam mosque and the Qutb complex, its original location, whether on the site itself or from elsewhere, is debated. The Iron pillar of Delhi is one of the most curious metal objects in the world. It was manufactured by the forge welding of pieces of wrought iron. In a report published in the journal Current Science, R. A critical corrosion-resistant agent called iron hydrogen phosphate hydrate makes the pillar resistant to rusting.
Experts at the Indian Institute of Technology have resolved the mystery behind the 1,600-year-old iron pillar in Delhi, which has never corroded despite the capital’s harsh weather.
Metallurgists at Kanpur IIT have discovered that a thin layer of “misawite“, a compound of iron, oxygen and hydrogen, has protected the cast iron pillar from rust.

The pillar bears an inscription which states that it was erected as a flagstaff in honor of the Hindu God and in the memory of the Gupta King Chandragupta II (375-413). How the pillar moved to its present location remains a mystery.
The question remains that how was such chemically advanced agent manufacture 2000 ago.
The pillar is a living testimony to the skill of metallurgists of ancient India