洞窟 has a slightly more literary or archaeological flavor than the everyday
山洞 (shāndòng, 'mountain cave') or
洞穴 (dòngxué, 'cave / cavern'). It's the standard term for the carved Buddhist cave-temples like
敦煌莫高窟 (the Mogao Caves at Dunhuang) and
龙门石窟 (the Longmen Grottoes).
