Mars Underground — William K. Hartmann

2032. The human race has established colonies on Mars. For years Dr. Alwyn Stafford researched its biggest mystery: Did life evolve on the Red Planet? The answer, except for simple, long-dead microorganisms, was no.

Now retired, Stafford stubbornly continues his quest. Rumors say he's been going farther than ever before into the Martian deserts.

Then he goes out and doesn't return. As the search for him grow, it becomes apparent that the old man found something that will forever change humanity's place in the cosmos...