Cavern

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

wiktionary

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From Middle English caverne, borrowing from Old French caverne, from Latin caverna(“hollow, cavity, cave”), from cavus(“hollow, excavated, concave”).

etymonline

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cavern (n.)

"large, natural cave under the earth," late 14c., from Old French caverne (12c.) "cave, vault, cellar," from Late Latin caverna "cave," from Latin cavus "hollow" (from PIE root *keue- "to swell," also "vault, hole"). In Old English such a land feature might be called an eorðscræf.