One of the  most ancient sites of rock-cut architecture in India are the Barabar hills located at a  distance 20-25km from Gaya town in the state of Bihar. They consist of four ancient caves called   Karan Chaupar, Lomas  Rishi, Sudhama and  Visva Zopri. They can be dated back to around 200 BC and were constructed during  the reign the great Mauryan king the Asoka for the  use of the Ajivika sages.

There are also beautiful cave temples inside the Barabar Hills. The construction of the caves influenced the building of the Buddhist Chaitya in Maharashtra as well as the Ajanta and Karla caves.

It has connections with the Mahabharata period. The great warrior of Mahabharata war, Ashwathama is said to be wandering even today in the valleys of Barabar. He had become mortal with the grace of Lord Sidhnath i.e. Lord Shiva, whose temple exists on the top of the highest mountain of Barabar hills.

In ancient times, Barabar was known as “KHALIT PARWAT” or “Nagarjuni parwat”. During Mahabhartha period, this was known “Gorthagiri” Either because the demon king Vanasur ruled here or because almost all the hills are of same height, this place came to be known as Banavar or Barabar in course of time. Vanasur was the senapati of the king Jarasandh and had constructed a fort on the hills, the remains of which can be seen even today.

Source:

http://jehanabad.bih.nic.in/impplaces.aspx

http://nalanda-onthemove.blogspot.in/2011/08/save-barabar-hillssave-buddhist.html