This well planned cave dewelling complex lies in the slopes of Mihintale. Kalu Diya literally means ‘black water’ a name derived from the shadow cast by the surrounding trees and boulders on the water making it appear black. The caves are well structured as living quarters with sleeping areas and sitting areas along with stone seats. The stone seats atop of rocks could have been used to preach to large gathering of monks or simply to meditate in the soothing breeze blowing across the pond.