Edgar Rice Burroughs,
The warlord of Mars
(1919)
“ For hours we followed the dark and gloomy river farther and farther into the bowels of Mars. From the direction and distance I knew that we must be well beneath the Valley Dor, and possibly beneath the Sea of Omean as well—it could not be much farther now to the Temple of the Sun. Even as my mind framed the thought Woola halted suddenly before a narrow, arched doorway in the cliff by the trail's side. ”
