How the massive tombs were built remains unknown.
Considerable mystery still surrounds an enormous and isolated megalithic site in southern India, called Hire Benakal, where scores of supposed tombs are constructed of upright slabs topped by capstones weighing 10 tons or more. Current conjecture is that the granite structures once contained graves, though no trace of human remains have been found there.
According to Archaeology.com:
The monuments were built over more than 1,000 years spanning the southern Indian Iron Age (1200-500 B.C.) and Early Historic (500 B.C.-A.D. 500) periods, and there are more than 1,000 of them across nearly 50 acres, from modest rock enclosures to mausoleum-like tombs.
Click here to see several photos of Hire Benakal and a brief description of it.