Vaibhar Hills are famous for two caves, peepal and saptaparni. The famous Peepal caves are present on the eastern slope of the Vaibhar hills. These caves are one of the oldest cave architecture in India. The Saptaparni Caves set on Vaibhar Hill were the venue of the First Buddhist Council, held to compile the teachings of the Buddha in its authentic form, after his death.
It is learned that there were plenty of hot springs in Rajgir earlier. Presently the most visited hot spring is in the eastern slope of Vaibhar hill. As there are multiple Hindu temple structures and tales related to Mahabharata with the main hot spring, it is very popular to Hindu pilgrims.
Source:
http://buddhistcircuitbihar.com/rajgir1.htm