In the courtyard of the Quwwat-ul-Islam Masjid is a 6.7m-high iron pillar that is much more ancient than any of the surrounding monuments. It hasn't rusted over the past 1600 years, due to both the dry atmosphere and its incredible purity. A six-line Sanskrit inscription indicates that it was initially erected outside a Vishnu temple, possibly in Bihar, in memory of Chandragupta II, who ruled from AD 375 to 413. Scientists are at a loss as to how the iron was cast using the technology of the time.