Caverns

The explorers ventured inside the caverns.

Source: tatoeba (2327349)

But, fearing this, the Sire omnipotent / hath buried them in caverns dark and deep, / and o'er them piled huge mountains in a heap, / and set withal a monarch, there to reign, / by compact taught at his command to keep / strict watch, and tighten or relax the rein.

Source: tatoeba (6779225)

"Ye know mad Scylla, and her monsters' yell, / and the dark caverns where the Cyclops dwell."

Source: tatoeba (6785020)

"And deep within the sides of pine / and caverns of the womb by stealth enclose / armed men, a chosen band, drawn as the lots dispose."

Source: tatoeba (6804491)