Iron Pillar (read below)
This pillar is one of the foremost metallurgical wonders of the world. It stands about 22 feet high and weighs more than 6 tons!!
"It was allegedly erected at the time of Chandragupta II Vikramaditya (375–413), though other authorities give dates as early as 912 BCE. It is the only remaining piece of a Hindu and Jain temple complex which stood there before being destroyed by Qutb-ud-din Aybak who built around it when he constructed the Qutub Minar and Quwwat-ul-Islam mosque.
The pillar is 98% wrought iron of pure quality, and is a testament to the high level of skill achieved by ancient Indian ironsmiths. It has attracted the attention of both archaeologists and metallurgists, as it has withstood corrosion for 1600 years, despite harsh weather."