Is it ensure, insure, or assure?
"a cave or cavity," late 14c., medical Latin, from Greek antron "cave."
antrum an·trum (ān'trəm)
n. pl. an·tra (-trə)
A nearly closed cavity or chamber, especially in a bone.
The pyloric end of the stomach, partially shut off during digestion from the cardiac end by sphincter muscles in the stomach wall.