The Mole People (1956) FFF

Archaeologists discover a mountaintop temple which leads them to the further discovery an underground civilization of Sumerians, lost to time and among whom they become trapped. This unknown, centuries-old subterranean patriarchal society comprises a brutal albino master race (having, over time, lost their pigment in their nearly lightless world), soil burrowing mole people (a subjugated humanoid slave race that has lost both human appearance and the power of speech), and goats (for milk, clothing, and occasional meat). All subsist on mushrooms. Their enclosed cavern limits their numbers; so, when three unexplained strangers arrive (the archaeologists), the high priest calls for their quick execution. A flashlight - with its indefensible brightness in their dim world - thwarts that plan, and three maidens must instead be sacrificed to the Eye of Ishtar, a temple chamber where citizens and criminals burn to death. Every once in awhile, someone within this society will be born with "the mark of darkness" (a.k.a. pigment). Thus is the case with Adad (whom everyone calls Adel), a genetic throwback whom the Sumerians easily give their visitors (now regarded as godly messengers due to the flashlight) as their own personal slave woman. Their presence, however, soon destabilizes this society, and as the mole people rise up to carry out their long-overdue revolt, the archaeologists are sentenced to death under the Eye of Ishtar - which turns out to be exactly what they want, since it's merely a fissure to the outside world. Taking Adel with them (the only current Sumerian able to tolerate sunlight), they regain the mountaintop, only to encounter an earthquake (seemingly the wrath of the goddess Ishtar objecting to Adel's escape).