The iron pillar of Delhi is a structure 7.21 metres (23 feet 8 inches) high with a 41-centimetre (16 in) diameter that was constructed by Chandragupta II (reigned c. 375–415 CE), and now stands in the Qutb complex at Mehrauli in Delhi, India. It is famous for the rust-resistant composition of the metals used in its construction. The pillar weighs more than 6 tonnes and is thought to have been erected elsewhere, perhaps outside the Udayagiri Caves, and moved to its present location by Anangpal Tomar in 11th century.
The height of the pillar, from the top to the bottom of its base, is 7.21 m (23 ft 8 in), 1.12 m (3 ft 8 in) of which is below ground. Its bell pattern capital is 306 mm (12 in). It is estimated to weigh more than six tonnes (13,228 lb). The pillar has attracted the attention of archaeologists and materials scientists because of its high resistance to corrosion and has been called a "testimony to the high level of skill achieved by the ancient Indian iron smiths in the extraction and processing of iron". The corrosion resistance results from an even layer of crystalline iron(III) hydrogen phosphate hydrate forming on the high-phosphorus-content iron, which serves to protect it from the effects of the Delhi climate.
The pillar carries a number of inscriptions of different dates, some of which have not been studied systematically despite the pillar's prominent location and easy access.[citation needed ]