QUESTION:
If we remove all the water from Earth and then looked at the surface, the
relative height would be pretty big.  As I recall, the Marianis trench is
something like 30 thousand feet deep and Mt. Everest is around 25 thousand
feet high.  That would make the differential height around 55 thousand feet? 
I have no idea how big the mountains are on Mars but they are being measured
from "sea-floor-level" in Earth terms.

Could you provide an accurate comparision of Mars/Earth with the assumption
that Earth has no "sea-level"?  I'm interested in what the numbers would
look like under these conditions.

ANSWER from Jeff Plescia on October 1, 1997:
The range in elevation on Mars is approximately 31 km:

Low point 	Hellas (- 4 km)
High point	Olympus Mons (+27 km)

On Mars the 0-km line is defined at the 6.1 mbar level in the atmosphere
(which is the pressure point of the triple point of water).

The relief range on Mars is about a factor of 2 greater than on the Earth.