How can the underground make world history ?

Candlelight or torchlight. Narrow holes for projecting the light. Carved-up letters in a lattice, or painted films.
Mirror and lens. The light of a fireplace may have been used in the side wall.
Voice, too, might have been used with megaphones or ducts.
Beside giving oracles, directed sound can have an impact upon a house or a body. Maybe it could be used to frighten people.