Quotation from: The Gods of Mars

Written by: Edgar Rice Burroughs

"I laboured with the other slaves a year since in the remodelling
of these subterranean galleries, and at that time we found below
these an ancient system of corridors and chambers that had been
sealed up for ages. The blacks in charge of the work explored
them, taking several of us along to do whatever work there might
be occasion for. I know the entire system perfectly.

"There are miles of corridors honeycombing the ground beneath the
gardens and the temple itself, and there is one passage that leads
down to and connects with the lower regions that open on the water
shaft that gives passage to Omean.