Same word as keyhole:

aperture, breach, break, burrow, cave, cavern, cavity, chamber, chasm, chink, cistern, cleft, covert, crack, cranny, crater, cut, den, dent, depression, dimple, dip, excavation, eyelet, fissure, foramen, fracture, gap, gash, gorge, hollow, hovel, keyhole, lacuna, lair, lea