source: By P199 - Own work, CC BY-SA 3.0

a cave, or a chamber in a cave, typically a large one.

大きな洞窟［ほら穴］.

used in similes and comparisons to refer to a vast, dark space:

広く暗い場所［部屋］

e.g. the dark cavern of the main performance hall

e.g. rouses me from the cavern of sleep.

late Middle English: from Old French caverne or from Latin caverna, from cavus ‘hollow’. Compare with cave1.