It seems that the first hallowed caves were dug during the time of The Sātavāhana Empire, which dates back to about 230BC.
Almost every surface but the floor is covered in paintings - literally. They have lost much of their former glory, of course, but efforts are being made to restore them. Poems are also painted on the walls - 547 of them - which tell stories of Buddha's previous lives.
To create them, a chisel was used to make room for plaster on the walls which was spread on the surface. Before it dried, the painter would paint on it, soaking it into the plaster so it became part of the surface. This method has been proven to survive over 2000 years.