What is a Dolmen?

A dolmen (also known as cromlech (Welsh), anta, Hünengrab, Hunebed, Goindol, quoit, and portal dolmen) is a type of single-chamber megalithic tomb, usually consisting of three or more upright stones supporting a large flat horizontal capstone (table). Most date from the early Neolithic period (4000 to 3000 BC). Dolmens were usually covered with earth or smaller stones to form a barrow, though in many cases that covering has weathered away, leaving only the stone "skeleton" of the burial mound intact.(Courtesy : Wikipedia)

The dolmen shown above is from Megalithic burial site Mallachandiram near Krishnagiri.
The Archaeology department has taken steps to preserve this 2000-year-old megalithic burial site — reckoned to be the largest in Tamil Nadu

More than 200 dolmens (tombs) of four types, dating back to the megalithic period (3 BC-3 Common Era), are found in the village. These include Cairn circles and tombs of migratory tribes. A majority of the dolmens were built of vertical slabs with portholes on the eastern side. Rectangular slabs, similar to railings, encircle these structures, which have passages made out of small rectangular slabs.

Inside the dolmens are paintings portraying human figures, bows and arrows, animals and symbols (Source The Hindu - http://www.thehindu.com/2006/10/04/stories/2006100404650600.htm)