The 7.21 meters tall structure is as much as 1600 years old and still stands completely rust-free.
Iron pillar of Delhi, India
Made from 98 per cent wrought iron, the pillar has been a subject of varied scientific studies from around the world. While a study concluded that critical corrosion-resistance agent called iron hydrogen phosphate hydrate makes the pillar resistant to rusting.
The pillar has attracted the attention of archaeologists and metallurgists as it has withstood corrosion for the last 1600 years, despite harsh weather and has been called "a testament to the skill of ancient Indian blacksmiths" because of its high resistance to corrosion. The iron pillar is one of the world's foremost metallurgical curiosities. The pillar, almost seven metres high and weighing more than six tonnes, was erected by Chandragupta II Vikramaditya (375-414 CE).
Made up of 98% wrought iron of pure quality, it is 23 feet 8 inches (7.21 m) high and has a diameter of 16 inches (0.41 m). Also, it was confirmed that the temperatures required to form such kind of pillars cannot be achieved by combustion of coal.
The pillar bears an inscription which states that it was erected as a flagstaff in honour of the Hindu god, Vishnu, and in the memory of the Gupta King Chandragupta II (375-413).
The pillar is a testament to the high level of skill achieved by ancient Indian iron smiths in the extraction and processing of iron.