QUESTION:
What are the highest and lowest elevations on the surface on Mars?

ANSWER from Smart Filter on 24 January 1997:
From the text "Astronomy" by James B. Kaler, copyright 1994, publisher:
Harper Collins:

First we must define a reference level by taking the average
atmospheric pressure at the surface of Mars (the reference level is
the martian counterpart of sea level on earth).  The volcano Olympus
Mons reaches 27 kilometers above the reference level.  The ancient
impact basin Hellas is 4 kilometers below the reference level.