Quotation from: The Gods of Mars

Written by: Edgar Rice Burroughs

"The therns, I judge from what you have said, are no less mortal
than we; and yet have I always heard them spoken of with the utmost
awe and reverence by the people of Barsoom, as one might speak of
the gods themselves."

"The therns are mortal," she replied. "They die from the same
causes as you or I might: those who do not live their allotted span
of life, one thousand years, when by the authority of custom they
may take their way in happiness through the long tunnel that leads
to Issus.