I hid the acorns in the hollow of the tree.
A squirrel lives in the hollow of the tree.
The cave in the mountain was very deep.
A hollow had formed at the base of the old tree.
洞窟 specifically means a cave large enough to enter, while 洞 can refer to any hollow, including small ones like a hollow in a tree.
洞 is a shorter, more casual way to say cave, while 洞穴 is the fuller, more standard everyday term.
洞（ほら）usually means a cave or larger hollow space one can enter, while 虚（うろ）is typically a smaller hollow, such as inside a tree trunk.
The kanji 洞 is composed of 氵(water) and 同 (same), originally referring to a water-filled hollow or cave. The reading ほら is a native Japanese word (kun'yomi) for a hollow or cave.