Is it ensure, insure, or assure?
c.1400, "full of caverns," from Latin cavernosus "full of cavities" (source also of Italian cavernoso, French caverneux), from caverna (see cavern). Meaning "hollow" is recorded from 1830.
cavernous cav·ern·ous (kāv'ər-nəs)adj. Filled with cavities or hollow areas; porous.