Analog Science Fiction - Science Fact May 1991 (Vol.111, No.6)
Cover by David Hardy (for "Intervention at Hellas")

Serial: "The Great Unknown" (Part 2 of 3) by Harry Turtledove (part of Earthgrip)

Novelettes: "Intervention at Hellas" by Daniel Hatch
"By a Waterfall" by Mark O. Halverson
"A Bargain at Any Price" by Grey Rollins

Science Fact: "Global Catastrophes and Mass Extinctions" by Jeff Hecht

Stories: "The Snows of Venus" by G. David Nordley
"Exodus" by Steven G. Johnson.

Stanley Schmidt's editorial is "Foolproof."