Lewis & Short

ŏpertānĕus, a, um, adj. [opertus], concealed, secret (post-Aug.): opertanea sacra, the secret rites of the Bona Dea, Plin. 10, 56, 77, § 156: di opertanei, who dwell in the bowels of the earth, Mart. Cap. 1, § 44